Maximum retries exceeded on transmission?


I have a strange problem: Sometimes (Approximetely 2 times a week) Calls are dropped unexpectedly

error message in my asterisk box:
Sep 17 09:16:37 WARNING[30001] chan_sip.c: Maximum retries exceeded on transmission B8728175-642411DC-BDF2CB3C-616AECB7@ for seqno 200 (Critical Response)
Sep 17 09:16:37 WARNING[30001] chan_sip.c: Hanging up call

My sip.conf contains the following lines:
Ports 10000-20000 and port 5060 are open.

I can’t use externip= line in sip.conf because my provider doesn’t support it. (i can’t register my voip numbers when i use this line…)

I’m using Asterisk 1.2.22

Can somebody help me please?

Thanks in advance!