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What's the Deal with Call Waiting Deluxe?

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Call Waiting Deluxe — it's one of the coolest phone features you've probably never heard of or used. But seriously — what is it and how does it work?

Call Waiting is something many of us take for granted these days, but of course, it hasn't always been around. Nowadays, thanks to voicemail and call waiting, it's actually become rather rare to hear a busy signal on the public phone network, which is a mixed blessing in that it has become easier to evade people. A busy signal tells you a very specific thing: I'm busy right now, but call back in a little bit. If you hear a line go right to voicemail right away, it's not really clear what the deal is. Was that Call Forwarding Busy? Safe Call Forwarding? Probably not Call Forwarding Don't Answer. You can't even trust audible ringback tone anymore - the line called may indeed be idle, or it might be in use. No way to know. (Not unless you have some niche CENTREX service like "Call Waiting Ringback Tone" - intragroup only, naturally.)

Call Waiting itself is preceded by Camp On, a similar but less sophisticated feature. Camp On provides an audible tone to both parties of a two-way conversation if another call was "camped on" that one. But you don't flash to answer the waiting call. There's no mechanism for that. Instead, once you hang up and the line is idle, the camped on call rings through. Like call waiting, there can be one camped on call at a time. Attendant camp-on was frequently used in CENTREX and in PBXs to allow a call to wait for a line to free up, saving the attendant the hassle of manually checking every so often and saying "Sorry, Mr. Smith, Mr. Jones is still busy." Now, Mr. Smith can wait himself until Mr. Jones becomes free.

Call Waiting, which first debuted with the Number One ESS, naturally extended this concept. On the 1ESS, the connections of a call are unbridged — physically — so that Call Waiting Tone (CWT) can be played only to the called party. Yet, unlike on today's digital switches, like the 5ESS or DMS-100, the other party knew that you had a Call Waiting. There were very noticeable clicks on the line when it was unbridged to play CWT to the party who had a waiting call. Evan Doorbell's "Centrex Quirks" series provides plenty of background about Call Waiting on the 1ESS, including how it integrated with the Call Hold feature, which, as Doorbell says, is "the feature nobody ever figured out how to use."

Doorbell himself provides a compelling argument for why the analog implementation of Call Waiting was, in some respects, superior to the digital one. On the 1ESS, the other party knew that you had a call waiting, because there were physical connections being unbridged and rebridged to allow for the CWT tone to be played privately. Thus, Call Waiting wasn't really a private affair. One amusing outcome of this was instruments that "faked" a Call Waiting be playing the clicks the other party would hear, with the hope that he might choose to end the call then and there rather than wait for you to put him on hold and come back. The more serious aspect of Call Waitings being a non-private affair was that there was less friction to attending to one. If you got a Call Waiting, the other party would be stopped mid-sentence, as it was unlikely he would miss the clicks. Thus, you could ask him to hold while you got your Call Waiting, and he knew exactly what was going on. Fast-forward to today, where the other party has no idea that you have a Call Waiting. Sure, it's private, but arguably ruder, too. Can't very well wait for a natural lull in the conversation, can you? Nope. "Er, um, hello, John? Yes, John? Can you hold please? I have another call? Yes, I have a call waiting, one moment please." The whole affair is very awkward and unnatural. Getting a Call Waiting on 1ESS is more like getting a knock on your door — you know you have a call waiting, and so does your client, and now that your conversation has been interrupted, you can ask him to hold a moment and attend to it. "Let the 1ESS interrupt your conversation for you, so you don't have to!"

Of course, the 1ESS interrupted calls this way for CWT because it had to, and this was all merely a side effect of it. And while this unintended benefit of it can be convenient, at least for people who usually attend to their Call Waitings, it can also be even more disruptive than Call Waiting on digital switches. However, there's no reason such bridging sounds can't be emulated on a digital switch. Just because it doesn't have to be done, doesn't mean it can't be, or even that it shouldn't be.

So, that's one way to make Call Waiting a bit nicer (or a bit worse, depending on who you ask). But wouldn't it also be nice if you knew who was calling?

Well, that's what Call Waiting Caller ID, or CWCID, is. Similar to how Caller ID works, with Bell 202 FSK being sent down the phone line after the first ring of an incoming call, for CPE (customer provided/premises equipment) to demodulate and show on a display, Call Waiting Caller ID allows you to see who an incoming call is from. And you don't even need to wait one ring — it shows up instantly. Unlike the earlier SFMF (Single Data Message Format), Call Waiting Caller ID provides both number and name, just like MDMF (Multiple Data Message Format). For more background on Caller ID, consult Sandman, but in a nutshell, Call Waiting Caller ID allows you to see who's waiting before answering, providing additional flexibility. (It's worth mentioning there are features like Talking Call Waiting, which don't require CPE, but TCW is even nicher and rarer than Call Waiting Deluxe.)

So, now you know who's calling, but what if you want to be able to choose how to respond to it?

Introduce… Call Waiting Deluxe.


Call Waiting Deluxe builds on Call Waiting Caller ID sort of. In theory, it's not inherently tied to CWCID; in practice, it is. CWD provides for disposition options that allow you to choose how to respond to a call waiting. There are five standard options:

  • Forward (to voicemail)
  • Drop (the current call, not the waiting call - to answer the waiting call)
  • Play a Hold message to the caller
  • Add, or conference, the caller into the existing call

The last option in particular is probably the most useful one. The other options are nice, but as long as Call Waiting has existed, you could flip back and forth between the calls but they always stayed separate. It doesn't have to be that way with the "Add" option, though! Say you're on the phone with a colleague and your boss calls to ask about the project you're both working on:

"Hi, Boss, yeah, Jim and I were just talking about that when you called. What's that? Have the parts arrived? I dunno, let me ask Jim".
FLASH.
"Hey, Jim? Yeah, that's the big boss calling. He wants to know if the parts have arrived."
"Parts? What parts?"
"I dunno, let me ask him."
FLASH.
"Hey boss, what parts are you talking about?"
"What do you mean what parts!? The parts Jim ordered yesterday."
FLASH.
"He says the parts you ordered yesterday."
"Well, I ordered two parts yesterday. And one of them has arrived and one of them hasn't, so it's a toss up."
FLASH.

(Oh, and that's all assuming that Call Hold isn't on this line. Otherwise, it's FLASH, followed by the three-digit Call Hold code each time. Call Waiting gives a whole new meaning to "finger exercises".)

Well! By now, perhaps, you can see how convenient it would be if we had just conferenced the boss in with Jim to begin with. Well, with Call Waiting Caller ID, you know the boss is calling. And with Call Waiting Deluxe, you can think "Hmm... he'll probably want to speak to both of us, not just me, let me conference him in." Thus, the Call Waiting Deluxe - Add/Conference disposition option is just like the exact opposite of Three-Way Calling. Instead of conferencing an outgoing call, you're conferencing an incoming call.

Now, the more technical part. How does it all work?

A precursory look into specifications for Call Waiting Deluxe will probably lead you to ADSI, or Analog Display Services Interface, a protocol that has sort of waxed and waned like the Ford Crown Victoria. ADSI uses the same Bell 202 FSK tones used for transmission of Caller ID, but it uses it for more than just Caller ID, and it can be used, theoretically, in both directions. US WEST made a few nice analog phones that were ADSI-capable. A telltale sign of ADSI support is 6 feature buttons, 3 symmetrically on each side of a display. If it's an analog phone (not digital, not VoIP), then chances are that's ADSI support right there. With ADSI, the switch can send a small program to the phone to dynamically change the menu options that appear next to the different buttons, and when you press one, you can communicate back to the switch what choice you want to pick. It's different from "press 7 to wait on hold for three hours" because a) the buttons are dynamic and not hard programmed to do only one thing and b) you probably don't have an ADSI-capable phone, so good luck if you want to use that program.

ADSI is still around today, with support even in Asterisk, at least for channel banks. One application of ADSI is "visual" voicemail, allowing you to navigate with buttons labeled on a screen, changing dynamically as you navigate the voicemail menus. Another application, supposedly, is Call Waiting Deluxe. Makes sense, right? There are 5 Call Waiting Deluxe options - one of them is holding the existing call and switching to the other, which works exactly the same way as it does without CWD. The other non-exclusive option is ending your existing call and switching to the incoming one, accomplished by hanging up and letting the waiting call ring through. The other three options are exclusive to CWD: forward the call to a predesignated destination, play a hold message, or conference the incoming call. Considering that the first of these will likely happen automatically and the second just makes you sound like a big shot call center, the latter is probably the most (and maybe only) useful option, but choice is good, and Call Waiting Deluxe gives you that choice. But not through ADSI - with DTMF.

Yup! That's right, folks! Hurry, hurry, step right up with your 10-button 1500 telephone set. Yes, you too, can take advantage of the great capabilities of Call Waiting Deluxe! And you don't even need ADSI support! Come again?

There's a reason dumpster diving behind central offices for documentation used to be common. It wasn't all that easy to come by information about how the system worked, and it's even worse today: Telecordia either asks for your first born or half of your liver if want a section of its documentation. Yet, there are a few subtle cues that ADSI is not very relevant, or at least not strictly necessary, for the operation of Call Waiting Deluxe. The first is an SBC practice from 2000, AM-TR-NIS-000112 to be specific. If you go to page 5, you'll notice something very strange if you thought that Call Waiting Deluxe was operated by bidirectional ADSI. It looks something like this:

  • DTMF 9 - FORWARD
  • DTMF 8 - ANNOUNCEMENT
  • DTMF 7 - DROP
  • DTMF 6 - HOLD
  • DTMF 3 - CONFERENCE

Wait a minute, Mr. Bell, what gives? ADSI is not DTMF. That's right — you don't actually need ADSI to signal which disposition option you want to choose. You use DTMF. So, yes, you could use a basic 1500 10-button TouchTone telephone. Heck, you could use a 500 rotary dial set with a pulse-to-tone converter in series with the line. The options are plentiful.

So, where does ADSI come into play, if at all? Well, this is just speculation, but a real CO with ADSI support would likely send a small ADSI program for Call Waiting Deluxe for the sets out there that are ADSI-capable. But, in reality, you don't need it. You don't need it, because ADSI is not used in the reverse direction - DTMF is - and the DTMF options are static and unchanging. So when a call comes in, if you have the options memorized or written down, it's fairly trivial to choose the option you want.

So, all this said, how does it work? Simple. A little experimentation with the BellSouth CI7112 Visual Director, one of the more advanced Caller ID display units - which also happens to have built-in Call Waiting Deluxe support - confirmed what a little research had suggested: DTMF is all that is needed to use the options. The bright blue button in the middle on the Visual Director is labeled "FLASH". Actually, all five of the buttons flash. The other four buttons flash and then play the appropriate DTMF tone. The FLASH button is just a pure general purpose flash button.

If you're trying to experiment with these buttons, BellSouth didn't make it easy. Perhaps that's a good thing. Since these are intended for use with Call Waiting Deluxe, and more specifically, because they perform a hook flash, they wanted to make sure you didn't use these buttons unless you actually had a Call Waiting. Otherwise, if you didn't have Consultation Hold (for Three-Way Calling), you'd accidentally disconnect a call. Once the unit gets the CAS (CPE Alerting Signal), a relay in the unit will operate to temporarily disconnect the connection to the phone so that it can receive the FSK for CWCID. Once it's done that, it rebridges the phone and you can see who's calling on the display. And, if you operate the four additional CWD-specific buttons, now they'll actually do something.

Naturally, the unit abstracts what it does, making it a bit cryptic to tell even then. A lineman's test set is handy in figuring some things out. If you have it connected to a switch under your control, rather than one owned by a phone company that probably gives so few damns it's probably detarriffed the Call Waiting Deluxe feature already, then you can spoof yourself a nice Call Waiting using an orange box to trick the unit into thinking there's a Call Waiting. Then, operate one of the buttons, and monitor the switch to see what happens. And, what happens? Just what the Ameritech practice from 2000 suggested: it sends a DTMF digit — after it flashes, of course. There is no special "Call Waiting Deluxe dial tone". The CI-7112 manual itself says to flash to switch calls, just like you would without Call Waiting Deluxe, but the other functions themselves function by means of a flash plus a DTMF digit. Of course, these digits shouldn't be confused with customer-dialed ones. So, there's a second of silence to receive a Call Waiting, and if a digit isn't received, you get a Recall Dial Tone like usual. All right, then! Three-way conferencing of an incoming call, here we come!

But, say you want to specify in advance what termination treatment an incoming call waiting should receive?

Introducing Advanced Call Waiting Deluxe. And now you finally know what *76 is for…

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