There are two numbers, for BT paging services
0345 010144 (300bd) 0345 010155 (1200/75)
There may be better speeds available now and you can also use PSS with 23422220102800.
You can have both modem and X.25 access to Mercury pagers too (one of the few areas where Mercury have actually managed to maintain any quality of service), and also at least modem access for Hutchison and Vodapage, possibly others.
A more computer oriented interface is implemented using the TAP (Telocator Access Protocol) service. Both Hutchison and BT use this. TAP has an American cousin called IXO, TAP is a superset of IXO.
Info from BT paging can be obtained by calling 0800 860860, ask for Technical Support and request them to send you doc ref PNO/600/0003, Technical Interface Spec for TAP. Someone reports success at producing a simple Visual BASIC interface.
The 'tpage2' package, with a few mods for BT specific bits will handle all the queuing of messages, and management of callout rotas, and the dialogue with the remote paging centre, etc.
'Sendpage' (<URL:ftp://ftp.gbnet.net/pub/paging/sendpage/>) runs on most Unix systems and will send pages via a TAP interface (and SMS messages).