Connected calls go to voicemail

I've had several occasions where I pick up an incoming call and see the counter start running (as in connected calls), but it becomes disconnected after a 3 or 4 seconds, and then I get a voicemail.

Can anyone confirm that this is a real bug in OS 7? I always thought calls that went to voicemail were supposed to go become missed calls, and you shouldn't be able to answer a missed call.

P.S. I have Google Voice integrated with Sprint but did not have this problem a Tour 9630.

I have never heard of or seen this before on any OS or BB to date. It's not an OS7 bug.