Is there any way to access the voicemail from the SDK. As I know this cannot be done. At least in the new SDK have they given any control over it?
You may not be able to directly access the voicemail from your app, but several operators provide a direct call number for people to dial directly into their voicemail.
You could then have people enter this voicemail number in the preferences and access it using a url like tel:1-408-555-5555
More information about this in the iOS Reference Library