The issue is that this 200 OK is not being ACKed:
Sent:
SIP/2.0 200 OK
Via: SIP/2.0/UDP aaa.bb.cc.40;branch=z9hG4bKfee8.d4965f1.0,SIP/2.0/UDP ddd.ee.fff.101:5060;received=ddd.ee.fff.101;rport=5060;branch=z9hG4bK-10765d90087576b04c489974689f47e7-ddd.ee.fff.101-1
From: "Anonymous" ;tag=ddd.ee.fff.101+1+3adf4e+dffad19e
To: <1234567890>;tag=B9D3DD8-1442
Date: Sun, 26 Sep 2010 18:32:24 GMT
Call-ID: 4CE9030C@ddd.ee.fff.101
CSeq: 64166520 INVITE
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, PRACK, UPDATE, REFER, SUBSCRIBE, NOTIFY, INFO, REGISTER
Allow-Events: telephone-event
Contact: <1234567890>
Record-Route: ,
Supported: replaces
Supported: sdp-anat
Server: Cisco-SIPGateway/IOS-12.x
Content-Type: application/sdp
Content-Disposition: session;handling=required
Content-Length: 219
v=0
o=CiscoSystemsSIP-GW-UserAgent 0 8513 IN IP4 10.198.16.1
s=SIP Call
t=0 0
m=audio 16848 RTP/AVP 0 101
c=IN IP4 10.198.16.1
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=ptime:201234567890>1234567890>
You will need to investigate this from the device that this 200 OK is being sent to, to find out a) if they receive this 200 OK and b) why they aren't sending an ACK for it.