[Serusers] 200 OK not being ACKnowledged

I'm testing SIP trunking connectivity to an ITSP through a Covergence (CXC)/ACME Packet session border controller. Calls from an IP phone to the PSTN through SER and the CXC are being dropped 15 seconds after the initial call is setup and just after a cal l is answered. We do not have this problem with calls which reach the PSTN via local gateways. It seems that SER is not relaying an ACK which it receives from the phone which causes the SBC to resend a 200 OK message until finally the connection times out. I've checked the call routing and do not see any issue however I do notice the following in the SER logs. Aug 13 13:07:24 vidar /usr/local/sbin/ser[19053]: [SER]: --ACK--: Time: [Thu Aug 13 13:07:24 2009] From: <sip:89390@vidar.net.isc.upenn.edu> To <sip:92155738396@vidar.net.isc.upenn.edu;user=phone> Src IP <128.91.2.223> Method:<ACK> Aug 13 13:07:24 vidar /usr/local/sbin/ser[19053]: [SER]: [Thu Aug 13 13:07:24 2009] [453a4bbf-90848c86-ea3a8091(a)128.91.56.18] <sip:92155738396@vidar.net.isc.upenn.edu:5060;user=phone;maddr=128.91.56.242;transport=udp> Starting PRESENCE/BLA section Aug 13 13:07:24 vidar /usr/local/sbin/ser[19053]: [SER]: [Thu Aug 13 13:07:24 2009] [453a4bbf-90848c86-ea3a8091(a)128.91.56.18] <sip:92155738396@vidar.net.isc.upenn.edu:5060;user=phone;maddr=128.91.56.242;transport=udp> End PRESENCE/BLA section Aug 13 13:07:24 vidar /usr/local/sbin/ser[19053]: [SER]: [Thu Aug 13 13:07:24 2009] [453a4bbf-90848c86-ea3a8091(a)128.91.56.18] --URI match--. Method: <ACK> R-uri: <sip:92155738396@vidar.net.isc.upenn.edu:5060;user=phone;maddr=128.91.56.242;transport=udp> Contact Header: <<sip:89390@128.91.56.18:5060>> From: <sip:89390@vidar.net.isc.upenn.edu> To <sip:92155738396@vidar.net.isc.upenn.edu;user=phone> IP source address <128.91.2.223> Aug 13 13:07:24 vidar /usr/local/sbin/ser[19053]: [SER]: --ACK--: Time: [Thu Aug 13 13:07:24 2009] From: <sip:89390@vidar.net.isc.upenn.edu> To <sip:92155738396@vidar.net.isc.upenn.edu;user=phone> Src IP <128.91.2.223> Method:<ACK> Aug 13 13:07:24 vidar /usr/local/sbin/ser[19053]: [SER]: Too many hops Aug 13 13:07:24 vidar /usr/local/sbin/ser[19053]: Warning: sl_send_reply: I won't send a reply for ACK!! User 89390 is a local subscriber and the phone initiating the call. The recipient of the call is my desk phone 215-573-8396 which is on a different, unrelated SER service. We do record_route traffic through SER for call control and accounting reasons. It seems like the ACK is looping through SER and therefore never being sent to the SBC but I cannot see why. Does anyone have any thoughts on this issue? Thanks,Steve