Can anyone help....

I have the Draytek 2750vn router, have tried all the settings i can think of but can not get VOIP to work properly.
The problem i have is i go to make a call firstly the ringing tone is not standard and secondly but more import both parties can not hear anything after answering the phone. Just a hiss noise that you get when you know your connected.

Hopefully someone can help

My voip provider is VOIPSTUNT

Also everytime I try make a call the VDSL (UK BT Infinity) connection drops and then reconnects.

I have tried two different providers VOIPSTUNT and VOIPBUSTER