We have people travelling internationally who are using different SIM cards for different countries on unlocked iphones. Different people use the SIMS so we don't want them to setup voicemail on each SIM each time they travel somewhere or hand off the phone.
So what I am doing is forwarding voicemail to my asterisk (trixbox) pbx. This can be done with RDNIS if your system supports it. The way I accomplished this though was through forwarding calls destined for voicemail to a specific DID I have on my asterisk system and setup the incoming route for the DID to go directly to the users office voicemail. They then get emailed the voicemail message.
Using the following call forwarding codes, I have the user forward voicemail calls to the DID I setup in asterisk.
Here is how I forward the calls on the cell phone (on the iphone, push and hold down 0 to get a + symbol):
No Answer Forward: **61*+ 1 DIDNUMBER # [SEND]
Unreachable Forward:**62*+ 1 DIDNUMBER # [SEND]
Busy Forward* * 67 * + 1 DIDNUMBER # [SEND]
This worked for me on AT&T.
