I've the same problem as you. And i've tried a lot of things to resolve this: running on a VPN, i've tried multiple router configs, as many routers asswell. In fact, i'm on the way to run an external sip account with a FXO gateway. It looks like impossible atm.
But a few time ago, i've just discover that there are differences betwen sip providers. And if you try another like (voipbuster) you could success on it.
S!
