Macbook air not picking up incoming SIP calls

I have tried multiple VOIP clients for Mac (telephone, x-lite, Jitsi etc.) and multiple SIP accounts. I can register and make outgoing calls with all. When I receive an incoming call I press "accept" in the relevant application, but the call never connects.

Does anyone else have this problem. I am using a Macbook air over Wifi.

(I have tried no headset, and the combined mic/phones jack, and have the same symptoms).