When I insert SIM in phone automagically configures stuff like voicemail number. As for voicemail, a local voicemail box is usually cheaper, and then you can implement your 'visual voicemail' too.

Example implementation:

1) Phone voicemail is on or off for A or B where A is GSM and B is SIP or VoIP. Can be multiple B. Can eventually even include support for IM phone calls, conferencing, or video chat.
2) If setting is on and phone is called for X seconds, them following is triggered: Phone is picked up, no microphone or speakers are enabled, message making clear voicemail is enabled is played to caller, input sound is recorded, data is flagged as voicemail in log. UI allows user to handle voicemail(s), including: play, reply, delete.

Basically like Eliza only works on Pidgin when its enabled, and when one is away...