I can reproduce this with Collabora's Freeswitch server and it eats all my battery and loads of bandwidth. So this is kind of critical.
Freeswitch side bug:
But from the log (I didn't investigate further), it looks like a bug in tp-ssip.
I've only tested with the version that is on the N900 which means sofiasip 1.12.10-0maemo5 and tp-ssip 0.6.2-0maemo4
I'm not sure how to reproduce it reliably..
This is in fact a sofia-sip bug.
A fix can be tested using this branch:
The fix is merged into sofia-sip master head.