Hi,
I just wanted to post this so that someone else might find the answer in future.
I have been having all sorts of problems after recently registering for a SIP line with Sipgate. REgistration was intermittant and sometimes I could nto make it work at all. After a LOT of digging (3 weeks working on it) I have found the solution is to DISABLE SIP ALG in the advanced settings. All SIP lines now work perfectly.
This is just a heads-up in case anyone else has SIP problems - I think there is an issue with the Dlink ALG for it. Other posts on the internet confirm this.