After some testing I see on SDP call stack after first INVITE,
.
U 2014/03/27 12:29:32.680559 10.237.236.150:56892 ->
10.237.236.207:5060
INVITE
sip:1200@networklab.loc;transport=UDP SIP/2.0.
Via: SIP/2.0/UDP
10.237.236.150:56892;branch=z9hG4bK-d8754z-4ac013f010b101fe-1---d8754z-.
Max-Forwards: 70.
Contact:
<sip:1240@10.237.236.150:56892;transport=UDP>.
To:
<sip:1200@networklab.loc;transport=UDP>.
From: "John
Couch"
<sip:1240@networklab.loc;transport=UDP>;tag=270d1828.
Call-ID: ZGNhMDdmOGQ1MWFhZTA2NGYwMTE1YThlM2UwNTQxYzI..
CSeq: 2 INVITE.
Allow: INVITE, ACK, CANCEL, BYE, NOTIFY, REFER, MESSAGE,
OPTIONS, INFO, SUBSCRIBE.
Content-Type: application/sdp.
Proxy-Authorization: Digest
username="1240",realm="networklab.loc",nonce="UzRTGFM0Uezw+Q1ZC/KUq4LDqr7BFZZ/",uri=
"sip:1200@networklab.loc;transport=UDP",response="0373158cea6615a4bca50401ace5af75",cnonce="4dfbe915fff3e822e9bc5ac7c3b526f8",nc=00000001,qop=auth,algorithm=MD5.
Supported: replaces, norefersub, extended-refer, timer,
X-cisco-serviceuri.
User-Agent: Z 3.2.21357 r21367.
Allow-Events: presence, kpml.
Content-Length: 165.
.
v=0.
o=Z 0 0 IN IP4 10.237.236.150.
s=Z.
c=IN IP4 10.237.236.150.
t=0 0.
m=audio 8000 RTP/AVP 0 101.
a=rtpmap:101 telephone-event/8000.
a=fmtp:101 0-15.
a=sendrecv.
U 2014/03/27 12:29:32.682376 10.237.236.207:5060 ->
10.237.236.150:56892
SIP/2.0 200 OK.
Via: SIP/2.0/UDP
10.237.236.150:56892;branch=z9hG4bK-d8754z-4ac013f010b101fe-1---d8754z-.
To:
<sip:1200@networklab.loc;transport=UDP>;tag=b27e1a1d33761e85846fc98f5f3a7e58.eb89.
From: "John
Couch"
<sip:1240@networklab.loc;transport=UDP>;tag=270d1828.
Call-ID: ZGNhMDdmOGQ1MWFhZTA2NGYwMTE1YThlM2UwNTQxYzI..
CSeq: 2 INVITE.
Server: kamailio (4.1.2 (x86_64/linux)).
Content-Length: 0.
.
U 2014/03/27 12:29:35.462838 10.237.236.150:56892 ->
10.237.236.207:5060
.