[Users] un-asked-for CANCEL being generated

[Mark Price]

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 at 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 at 66.129.95.5>;tag=as11c24889..Call-ID:
1a095b0e1901cead54e8d68c63271aad at 66.129.95.5..To:
<sip:9043067833 at joinuneta.com:5065>..CSeq: 102 CANCEL..User-Agen
  t: OpenSer (1.1.0-tls (i386/linux))..Content-Length: 0....