Calling another blackberry - rings normal then goes to a "fast ring"
 

Calling from a blackberry to a blackberry, it rings normal, then goes to a "fast ring". The receiver has voicemail as well. The fast ring, is like a normal ring with no pauses?
Occurres on a Rogers network, very infrequently.
What is it? - if the receipient is on the blackberry, why doesn't it go to vm?

A fast tone indicates network congestion, not a problem with the phone specifically, and your call isn't even making it to his phone to be routed to voicemail. It is being blocked by the system before that.