Voice Mail Setting

Hello All!
I am now at the point in my deployment where I am “working out little things” witch is great! my question is :

When I click on the voicemail button on my sip phone. It attempts to dial to asterisk@ is the correct IP address)
I am then greeted with a congestion tone.
I called the phones maker and we set anything we could to point to the correct extension. Yet i have this feeling that it is hiding in one of the config files in *. Please help !
Thank You ,