The Linux Modules for connection Tracking are making problems if they are run on the same machine as asterisk.

• The Symbian (E61, ...) Sip client istn't working ( See also Comment "Strange Problems with Netfilter Modules" in Nokia )
• On some incoming calls only one side can understand something (the IP Address of the Peers is changed from the module, so Asterisk is streaming to Nirvana)

You can deaktivate the Modules permanently ba changing the
/etc/modprobe.conf
to
install ip_nat_sip /bin/true
install ip_conntrack_sip /bin/true