I had the same problem (3 accounts at Sipphone.com) and I finally eliminated it be using the following entery in sip.conf:
With just this entry in sip.conf and my 3 register => statements in sip.conf I get calls in to my extensions file where they are matched against Sipphone's fake number (begins with 1747...)
Unfortunatley I can no longer send outbound calls to Sipphone as I had to remove the following outbound entry in sip.conf:
The sip.conf entry above was catching all the inbound INVITE messages from Sipphone and asking them to authenticate themselves (407) after which the call would disapear.
I signed up with Draytel for my outbound calls.
I hope we get a better soultion to thie problem from someone....