I am using the fritzbox iphone telephony app but I've never been able to get it to work reliably. 

It might work for a while, but, then, I start getting the following error on incoming calls...

22.08.13 15:27:15 Internet telephony with 620@;ob via;ob failed. Cause: Decline (603)

The app will ring very briefly then disconnect before I've had a chance to answer. 

Any ideas?