Sip trunk between two asterisk pbx


I had a working sip trunk between two asterisks pbx. Last week
I did a system update from fedora 10 to fedora 11 only on one
system. After the system upgrade the sip trunk does not work.
Old system (Fedora 10) has this asterisk version: asterisk-
New system (Fedora 11) has this asterisk version: asterisk-
The strange is that the connection does not work only from one direction but
from the other direction the connection works. Here is the topology:

SIP-phones—IP—AsteriskA —IP----AsteriskB—analog—ISDN-PBX—simple-PSTN-phones

the connection works: simple-PSTN-phones —>SIP-phones
the connection does not work: SIP-phones —> simple-PSTN-phones.

The sympthom:
The simple PSTN-phone starts ringing but after 2,3 rings I got a disconnect
signal at SIP-phone side… It seems that the SIP part wokrs but the RTP does not… Please help. If you need more information I can send, I just did not want to flood one post.

Thanks in advance,