My E65 is finally back from repairs and I managed to get hold of a different router (Fritzbox, can't remember model name right now, will update later) which allows me to connect and use the VOIP...

My E65 is currently being repaired (faulty display) so I don't really have any big updates but I'm pretty sure I had filled in all details and it still didn't work.
In fact, I had set up an asterisk...

I can very much recommend asterisk. It's easily configurable and also gives VoIP debugging output. Plus it's free & open source.

Update:
The same behaviour can be seen when trying to connect to internetcalls.com
While testing this additional VoIP service, I encountered another weird issue:
when there is more than one SIP...

I'm trying to connect my E65 to the sipgate.de VoIP service using the built-in client, however it seems to be unable to establish a connection. I started investigating the issue a bit myself and...