My previous Windows Mobile phones had a field in the phone’s settings that let you forward unanswered calls to another number. This let me use my PBX’s voicemail instead of AT&T’s voicemail so I only had to deal with one voicemail system.
The iPhone 3G doesn’t have this field in the phone’s settings (at least I can’t find it). The workaround involves placing a call to a special phone number. If the number of the phone number which you want to forward calls to is 310-555-1212 place a call to this number (include the stars and pounds):
**67*13105551212#
This works for AT&T and more information can be found here and here.
