[neohaven]

> There doesn't seem to be a +11~, but is it forbidden?

Well, kinda... The thing is, even if not required by standard (It is, there's a Numbering Plan for the US and Can), most 9-to-dial-out systems would fail to call such a number, redirecting 911-anything to immediately dial out 911 once the second 1 is dialed.

IIRC, I've seen POTS also skip the inter-digit tone wait (the time between your last dial and the system going "okay, that's all the numbers, let's call!") when calling x11 numbers, generally considering "911" and other emergency/service numbers to be a magic triple that doesn't wait for other numbers to dial out.

There are no area codes starting with 1 in the US nor Canada to my knowledge since any dialing in a system with 9-to-call-out should end up being dialed as follows :

* 9 (leave system)

* 1 (country code)

* 1 (first digit of the area code)

And there is no step 4, because 911 is a special number like 411 and 311, that has a special dial plan. Since there is a possibility of issues if we make the treatment any less dumb, we don't do that. 999 is a special, unassigned code in the US too. 112 is likewise impossible to clash with here in NA since calling that with a corp system just means typing 9112, which ends up being 911, and dialing it on its own can also be allowed to be a special case since there are no area codes with 1 because of 911.

In fact, even "small area codes" or, officially, "office codes" are also required to not be X11. You used to be able to call locally in your area code without dialing the area code, too. It ends up being that xxx-911-xxxx is also an invalid number. Because locally, you would have had to type "911" to start dialing that number. There are also other limitations that can be seen here as well as allocation for future expansion : https://en.wikipedia.org/wiki/North_American_Numbering_Plan#... https://en.wikipedia.org/wiki/North_American_Numbering_Plan_...

There's also probably a physical, legacy reason it's probably much more difficult to make a physical 911 switch when "911" is also a valid part of a phone number. It means you need to figure out "Is it what they meant, or are there more numbers coming?" It means the sequence of operations "<open line><9><1><1>" always means emergency, fast connect, and probably easier to implement as a side-channel than as a part of the dial logic? This is speculation, I haven't actually opened a POTS PBX with a physical implementation of the 911 dialing logic.