I've been using GV since June/July 2010. It was working fine until 1-2 months ago. I don't really know when it started acting up since I don't really get that many calls and not everyone that does call would report the issue. Anyway, issue in question is that when someone calls my mobile's number, they get a prompt to enter the number they wish to dial and press pound key. If they do this then it takes them to my google voicemail.
To fix this I have tried re-installing the application, I even rebooted to recovery, wipe EVERYTHING, and installed my custom rom. During the GV installation, it ask to dial to dial *28850.... After I do that and hit next, it goes to my phone voicemail settings menu and reports "Failed to retrieve and save current forwarding number settings. Do you want to switch to the new provider anyway?". I suspect this is the issue. That Sprint forwarding to GV isn't working which is why it prompts to enter the number you wish to dial.