This is fascinating. It never occurred to me until now that there might be a window in the call process between "line is flagged as busy" and "phone rings."

I assume we're talking about a very narrow window here, right? How narrow?

Is it something that has always existed, or something that crept in because of the transition from phone-as-mechanical-device to phone-as-software?

As long as it's a fairly consistently timed window (and I assume it is), it doesn't matter if it's just a few milliseconds.

On a related note, I'm still surprised to find that telephone operators haven't fixed the scenario "someone calls me and I just miss it, I call them back while they're trying to call me for the second time, and we both get a busy signal". As a naive outsider, seems like it wouldn't take much to just connect those two calls.

Under that scheme, if I called my own line by dialing my own DID, I'd end up with a completed loopback call rather than a connection to my voicemail inbox.

Just have an exception for that case?

I don't know how phone systems work, but in any other programming scenario, a quick if..else would fix that.

To translate into Internet-engineering terms: assume that you might be having to call out through a proxy server (this is true when you e.g. have to "dial 9" to reach an outside line.) If your own phone is public-routable (i.e. your extension has its own DID, and that's what you're calling), then the routing system outside the device will have no idea that what you'll reach is, in fact, your own device. The origin of the call will appear to the POTS system as just "the PBX" (a NAT, basically); and the destination will appear to your PBX as just some anonymous public number owned by a phone company, that happens to proxy back to your own extension on the PBX when you request it. So neither the POTS system nor your PBX will be able to recognize what's going on. (I mean, you can manually program associated numbers into the PBX, but by doing so you're denormalizing DID mappings: they'll break as soon as someone's number changes on the POTS side.)

Much better if dialing out on a line just takes an exclusionary lock on the line so that it can't be dialed into; then anything that forwards back to that line, at any level of indirection, will be told the line is busy and will step to the next part of the call-tree.

(Note that this becomes important when you're trying to e.g. get a [random peer of your support-level] on the line for a consult in tech support, by "adding" to the call the same ring-group you're in. Exclusionary locking here is what ensures that you won't end up adding yourself to the call.

Perfect explanation, thank you!

>As a naive outsider, seems like it wouldn't take much to just connect those two calls.

Who would you charge for the call?

All phones services I've used charge for total usage, regardless of the initiator. If someone calls me, I still get dinged for every minute I'm on the line, same as if I placed an outgoing call. (Granted, I've only used pay-as-you-go type services, so perhaps other services are different.)

In Europe all services I have seen has been of the kind where whoever makes the call pays, and nobody ever gets charged for receiving calls.

Under such a system, this can become a issue.

I bet from the phone company they don't see the problem. Both were intending to place a call so they both pay :)

Apparently only the US and Canada charge the phone owner for incoming calls.

https://en.wikipedia.org/wiki/GSM_services#Incoming_calls

Europe, Asia, Australia etc all charge the caller.

This leads to awkward situations like when I called someone in the US and they were worried that they're going to get charged international rates for a call from abroad.

Wow, dick move. Getting charged for the incoming call is something I associate with getting ripped off by roaming call charges.

Many years ago, using Virgin Mobile (in the UK) I remember being charged for incoming calls, however these days on all UK networks (and I assume this is the case for most of Europe) the caller gets charged but the callee does not

Both of them, obviously. ;)

The one with lower timestamp.

I hear the crackling of my PC speakers a few seconds before the call rings on my mobile; similarly on our landline the phone lights up and shows the caller ID a second or so before it rings.

Also if you ring a phone in the same room as you [we have kids!] you'll often find you get a ring tone long before the callee's phone rings.

> on our landline the phone lights up and shows the caller ID a second or so before it rings

Not sure about the US, but in Australia this is a phone specific feature. The Caller ID comes between the first and second ring, so if you want a call blocking feature that is completely silent, you need to silence the first ring of every call. Some phones will do this automatically, others (like my Panasonic landline) make it an option you can turn on & off. The Panasonic FAQ has some info:

http://eng.faq.panasonic.com/app/answers/detail/a_id/26547/~...

I'm in UK, cheap German portable landline phone with virtually no useful settings. Could well be phone specific as you suggest.

Could be a second or two. The traditional phone systems are dog slow. Often takes 15 secs from A starts ringing to B does.

I can confirm. At work I have to deal with call processing [1], and the time guard [2] is 15 seconds on the originating side [3] for CDMA [4].

[1] We provide enhanced caller ID---a name in addition to the phone number. Our new product will also mark known telemarketers.

[2] The amount of time we have to do what we do

[3] The calling party---the one initiating the phone call.

[4] which may be enough to identify the carrier we work with.