Saturday January 31, 2009 Random ramblings
Saturday January 31, 2009 Note to phone system operators: Relative format caller-ID is evil
Why, oh why, do phone operators configure their systems to send relative phone numbers as the caller-ID? Which muppet made that decision? 
Ever since countries, under the ITU, agreed to a common international, direct-dial numbering standards, phone numbers share a global, hierarchical number-space. This number-space is rooted (notionally) at "+", and leads to numbers like "+44 141 bcd efgh". Such a number is "fully-qualified" and uniquely identifies some port to the phone system, globally. E.g. this example is a number in the UK, nominally in Glasgow (the 141 code). One little nit is that the "+" can translate to different codes in different systems. Thankfully many countries use "00"; further, GSM phones can just dial "+", it seems (exactly how this works I don't know, but it seems to work with multiple phones on multiple networks in various countries - presumably it's part of the standard).
It also possible to have "relative" phone numbers, which assume part of the prefix (aka Subscriber Trunk Dialling) - alternatively, you can think of this as "bottom up" or "local" dialling. E.g. if one is in the UK, there is no need to dial +44, just dial "0141 bcd efgh"; if one is in Glasgow (and certain other localities) there is no need to dial the 141 part, just dial "bcd efgh"; if one is dialing from a phone sharing the same bcd part as its primary number then that possibly can be dropped too (an exchange-local call, though the "bcd" part need not uniquely identify an exchange), just dial "efgh".
So basically a fully-qualified number can be dialled from any public phone system in the world, and it should just work, while a relative number only works in certain places. The problem is that many operators today send Caller-ID in relative form. E.g. if someone rings my UK mobile, I will see something like "078a bcd efgh" or "0141 bcd efgh" on my phone's display. This lead to various annoyances, not least:
- If I travel to another country and then attempt to ring them back, it fails. (It will hopefully fail by telling me its an invalid number, however it's possible the number will be valid in that other country, and I get to phone some random person).
- My mobile phone defaults to using VoIP when available, which just happens to be in another country to the one I'm usually in. So if I ring back when connected to wifi I get fail (hopefully an invalid number error, but see previous).
- If I try to forward such a number (SMS, bluetooth, etc) to someone whose phone dials from another country, one of us needs to remember it's in relative form and edit it to make it fully-qualified.
All these problems would be avoided if phone networks just sent fully-qualified, E.164-form numbers as their Caller-ID. Relative numbers == inconvenient == fail == evil.
Even more helpful are BT: BTs landline network disallows E.164 dialling! You can't dial "+44 141 bcd efgh", (you can't even dial "0141 bcd efgh" iirc if in Glasgow). This is a big, stinking pile of FAIL if you've got a home VoIP/POTS router and you want it to dynamically select whether to dial-out via BT or via VoIP - you need to apply a bunch of horrid number-rewriting rules to stand a chance of this working, and lower-end VoIP/POTS routers might not even be capable of doing so. I can't understand why BT would disallow fully-qualified dialling to UK numbers.. BT just suck. (I think this works fine with Eircom in Ireland).
To summarise:
- Relative numbers == legacy == inconvenient == fail == evil.
- E.164 Caller-ID == kittens & hugs.
- Disallowing E.164 dial-out == fail == BT
Trackback URL: http://blogs.sun.com/paulj/entry/note_to_phone_system_operators
Post a Comment:
BT *do* allow full national code dialling for local numbers, though you are correct that they don't allow dialing the 0044 code for the UK *from* the UK.
Posted by Andy Burns on February 01, 2009 at 11:30 AM GMT #