Call Queue to Voicemail Issue

Hi Guys.. I am still getting a problem with my call Queues and i am now getting a bit desperate.. PLEASE! can someone ppoint me in the right direction. 

We have several Call queues set up and when a call 'times out' currently set to 15 seconds it should got to a users account.  but what we are finding is that the call gets to the queue, redirects but then nothing,  simply silence.... but if i call the user directly its fine