You would need to see a response to the REGISTER.
Here is a typical successful registration.
67687mS SIP Tx: UDP xxx.xxx.xxx.xxx:5060 -> 70.167.153.130:5060
REGISTER sip:sip.flowroute.com SIP/2.0
Via: SIP/2.0/UDP xxx.xxx.xxx.xxx:5060;rport;branch=z9hG4bKab5ebf43f75909e8e99a36e87bfc3821
From: <sip:xxxxxxxxx@sip.flowroute.com>;tag=b1d2c6f874d5c2b1
To: <sip:xxxxxxxxx@sip.flowroute.com>
Call-ID: 242f104fbb36bbe027239bbfce4f2d71@xxx.xxx.xxx.xxx
CSeq: 784319672 REGISTER
Contact: "Unknown" <sip:xxxxxxxxx@xxx.xxx.xxx.xxx:5060;transport=udp>
Expires: 3600
Max-Forwards: 70
User-Agent: IP Office 5.0 (4)
Content-Length: 0
67760mS SIP Rx: UDP 70.167.153.130:5060 -> xxx.xxx.xxx.xxx:5060
SIP/2.0 100 Trying
Via: SIP/2.0/UDP xxx.xxx.xxx.xxx:5060;rport=5060;branch=z9hG4bKab5ebf43f75909e8e99a36e87bfc3821
From: <sip:xxxxxxxxx@sip.flowroute.com>;tag=b1d2c6f874d5c2b1
To: <sip:xxxxxxxxx@sip.flowroute.com>
Call-ID: 242f104fbb36bbe027239bbfce4f2d71@xxx.xxx.xxx.xxx
CSeq: 784319672 REGISTER
Content-Length: 0
67762mS SIP Rx: UDP 70.167.153.130:5060 -> xxx.xxx.xxx.xxx:5060
SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP xxx.xxx.xxx.xxx:5060;rport=5060;branch=z9hG4bKab5ebf43f75909e8e99a36e87bfc3821
From: <sip:xxxxxxxxx@sip.flowroute.com>;tag=b1d2c6f874d5c2b1
To: <sip:xxxxxxxxx@sip.flowroute.com>;tag=e9901e48385c1aec25ff857f21344e5a.d027
Call-ID: 242f104fbb36bbe027239bbfce4f2d71@xxx.xxx.xxx.xxx
CSeq: 784319672 REGISTER
Digest realm="sip.flowroute.com", nonce="4a5dcbcc0001147b6660739b77f028f8fbac9fbc11928eec", qop="auth"
Content-Length: 0
67765mS SIP Tx: UDP xxx.xxx.xxx.xxx:5060 -> 70.167.153.130:5060
REGISTER sip:sip.flowroute.com SIP/2.0
Via: SIP/2.0/UDP xxx.xxx.xxx.xxx:5060;rport;branch=z9hG4bK5966ff5e1f12925f997684909b51cdbf
From: <sip:xxxxxxxxx@sip.flowroute.com>;tag=b1d2c6f874d5c2b1
To: <sip:xxxxxxxxx@sip.flowroute.com>
Call-ID: 242f104fbb36bbe027239bbfce4f2d71@xxx.xxx.xxx.xxx
CSeq: 784319673 REGISTER
Contact: "Unknown" <sip:xxxxxxxxx@xxx.xxx.xxx.xxx:5060;transport=udp>
Expires: 3600
Authorization: Digest username="xxxxxxxxx",realm="sip.flowroute.com",nonce="4a5dcbcc0001147b6660739b77f028f8fbac9fbc11928eec",response="0496b0d9add08325c318225bd5e2c02e",uri="sip:sip.flowroute.com",algorithm=MD5,qop=auth,nc=00000001,cnon Max-Forwards: 70
Max-Forwards: 70
User-Agent: IP Office 5.0 (4)
Content-Length: 0
67841mS SIP Rx: UDP 70.167.153.130:5060 -> xxx.xxx.xxx.xxx:5060
SIP/2.0 100 Trying
Via: SIP/2.0/UDP xxx.xxx.xxx.xxx:5060;rport=5060;branch=z9hG4bK5966ff5e1f12925f997684909b51cdbf
From: <sip:xxxxxxxxx@sip.flowroute.com>;tag=b1d2c6f874d5c2b1
To: <sip:xxxxxxxxx@sip.flowroute.com>
Call-ID: 242f104fbb36bbe027239bbfce4f2d71@xxx.xxx.xxx.xxx
CSeq: 784319673 REGISTER
Content-Length: 0
67843mS SIP Rx: UDP 70.167.153.130:5060 -> xxx.xxx.xxx.xxx:5060
SIP/2.0 200 OK
Via: SIP/2.0/UDP xxx.xxx.xxx.xxx:5060;rport=5060;branch=z9hG4bK5966ff5e1f12925f997684909b51cdbf
From: <sip:xxxxxxxxx@sip.flowroute.com>;tag=b1d2c6f874d5c2b1
To: <sip:xxxxxxxxx@sip.flowroute.com>;tag=e9901e48385c1aec25ff857f21344e5a.e5e8
Call-ID: 242f104fbb36bbe027239bbfce4f2d71@xxx.xxx.xxx.xxx
CSeq: 784319673 REGISTER
Date: Wed, 15 Jul 2009 12:29:35 GMT
Contact: <sip:xxxxxxxxx@xxx.xxx.xxx.xxx:5060;transport=udp>;q=0.1;expires=3600
Content-Length: 0
Notice the final result is receiving a 200 OK
Kyle Holladay
ACA-I, ACA Call Center, ACS-I, ACS-M, TIA-CTP, MCP/MCTS Exchange 2007
ACE Implement: IP Office
"Thinking is the hardest work there is, which is the probable reason why so few engage in it." - Henry Ford