[Users] un-asked-for CANCEL being generated

I'm using openser 1.1.x, and seeing something rather strange. We are using openser as a proxy with openser and asterisk on box A, asterisk on box B. Openser listens on ports 5060 and 5065. Asterisk listens on port 5070 on box A and 5060 on box B. When a call arrives from PSTN, it works fine if I forward to asterisk in box B. If I forward to asterisk on box A, everything seems to be going well and then openser generates a CANCEL to the receiving phone for seemingly no good reason. You can see from the packet below that the cancel is being generated by openser, because of the user agent. What might cause this? Thanks, Mark Price U 66.129.95.24:5065 -> 71.199.89.40:16742 CANCEL sip:9043067833@71.199.89.40:16742;rinstance=f5f4de634aa6bff6;transport=udp SIP/2.0..Via: SIP/2.0/UDP 66.129.95.24:5065;branch=z9hG4bK1c31.2cb72f11.0..From: "Mark Price" <sip:9046268172@66.129.95.5>;tag=as11c24889..Call-ID: 1a095b0e1901cead54e8d68c63271aad@66.129.95.5..To: <sip:9043067833@joinuneta.com:5065>..CSeq: 102 CANCEL..User-Agen t: OpenSer (1.1.0-tls (i386/linux))..Content-Length: 0....