Perhaps a race? Some numbers work, others not

Running ekiga on linux some numbers just silently disconnect while
others work. Calling the same numbers from my Grandstream ATA always
works.

This is using sound only: no video.

A simple example is calling the sipgate test number [email protected].
That *always* works. In contrast calling [email protected] (their
voicemail number) usually disconnects silently.

However if I set -d 4 for debug, [email protected] almost always
works. Which is why I suspect a race. 

I have managed after many attempts to capture a debug trace of
[email protected] failing. I also captured a [email protected]
debug trace working. A simple diff of the two cases wasn't useful (for
me: far too much noise, and the traces differed almost every where).

The two gzipped traces are 41K and 48K long so quite compact. 
Any help with debugging these?

ael