Something

asterisk1*CLI> sip debug ip 192.168.253.3
SIP Debugging Enabled for IP: 192.168.253.3
asterisk1*CLI>
<-- SIP read from 192.168.253.3:5060:
REGISTER sip:192.168.253.15 SIP/2.0
Via: SIP/2.0/UDP 192.168.253.3:5060;rport;branch=z9hG4bK49c23c2489f34b477bcabc34bcd46cf6
From: <sip:useip@192.168.253.15>;tag=7409e9d3
To: <sip:useip@192.168.253.15>
Contact: <sip:useip@192.168.253.3:5060>
Call-ID: 16b6e1f02ab15a244b9b024f17bab23e@192.168.253.3
CSeq: 2 REGISTER
Max-Forwards: 70
Expires: 300
Allow: INVITE,CANCEL,ACK,BYE,NOTIFY,REFER,OPTIONS,INFO,MESSAGE
User-Agent: CMI CM5K
Content-Length: 0


--- (12 headers 0 lines)---
Using latest REGISTER request as basis request
Sending to 192.168.253.3 : 5060 (NAT)
Transmitting (NAT) to 192.168.253.3:5060:
SIP/2.0 404 Not found
Via: SIP/2.0/UDP 192.168.253.3:5060;branch=z9hG4bK49c23c2489f34b477bcabc34bcd46cf6;received=192.168.253.3;rport=5060
From: <sip:useip@192.168.253.15>;tag=7409e9d3
To: <sip:useip@192.168.253.15>;tag=as0582e7ad
Call-ID: 16b6e1f02ab15a244b9b024f17bab23e@192.168.253.3
CSeq: 2 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Contact: <sip:useip@192.168.253.15>
Content-Length: 0


---
Scheduling destruction of call '16b6e1f02ab15a244b9b024f17bab23e@192.168.253.3'in 15000 ms
Destroying call '16b6e1f02ab15a244b9b024f17bab23e@192.168.253.3'
asterisk1*CLI>
<-- SIP read from 192.168.253.3:5060:
REGISTER sip:192.168.253.15 SIP/2.0
Via: SIP/2.0/UDP 192.168.253.3:5060;rport;branch=z9hG4bK5a3afdc739d8f3818601239c2f7f077f
From: <sip:useip@192.168.253.15>;tag=7de36ab1
To: <sip:useip@192.168.253.15>
Contact: <sip:useip@192.168.253.3:5060>
Call-ID: 08bae2e755a639c24439a51354a42efa@192.168.253.3
CSeq: 2 REGISTER
Max-Forwards: 70
Expires: 300
Allow: INVITE,CANCEL,ACK,BYE,NOTIFY,REFER,OPTIONS,INFO,MESSAGE
User-Agent: CMI CM5K
Content-Length: 0


--- (12 headers 0 lines)---
Using latest REGISTER request as basis request
Sending to 192.168.253.3 : 5060 (NAT)
Transmitting (NAT) to 192.168.253.3:5060:
SIP/2.0 404 Not found
Via: SIP/2.0/UDP 192.168.253.3:5060;branch=z9hG4bK5a3afdc739d8f3818601239c2f7f077f;received=192.168.253.3;rport=5060
From: <sip:useip@192.168.253.15>;tag=7de36ab1
To: <sip:useip@192.168.253.15>;tag=as67260d30
Call-ID: 08bae2e755a639c24439a51354a42efa@192.168.253.3
CSeq: 2 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Contact: <sip:useip@192.168.253.15>
Content-Length: 0


---
Scheduling destruction of call '08bae2e755a639c24439a51354a42efa@192.168.253.3'in 15000 ms
asterisk1*CLI>