<--- SIP read from 192.168.1.100:5109 --->
SUBSCRIBE sip:1100@asterisk SIP/2.0
Via: SIP/2.0/UDP 192.168.1.100:5109;branch=z9hG4bKac56bb7c6f120d6c215f0cf371824380;rport
From: "1100" <sip:1100@asterisk>;tag=2255010868
To: "1100" <sip:1100@asterisk>
Call-ID: 2707663927@192_168_1_100
CSeq: 918274845 SUBSCRIBE
Contact: <sip:1100@192.168.1.100:5109>
Max-Forwards: 70
User-Agent: S450 IP020970000000
Event: message-summary
Expires: 3600
Allow: NOTIFY
Accept: application/simple-message-summary
Content-Length: 0

<--- Transmitting (no NAT) to 192.168.1.100:5109 --->
SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP 192.168.1.100:5109;branch=z9hG4bKac56bb7c6f120d6c215f0cf371824380;rport;received=192.168.1.100
From: "1100" <sip:1100@asterisk>;tag=2255010868
To: "1100" <sip:1100@asterisk>;tag=as534c92bc
Call-ID: 2707663927@192_168_1_100
CSeq: 918274845 SUBSCRIBE
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
WWW-Authenticate: Digest algorithm=MD5, realm="asterisk", nonce="4013b650"
Content-Length: 0

<--- SIP read from 192.168.1.100:5109 ---> line 1390
SUBSCRIBE sip:1100@asterisk SIP/2.0
Via: SIP/2.0/UDP 192.168.1.100:5109;branch=z9hG4bK3429f3c23251b17ed5f698e1c2f556ec;rport
From: "1100" <sip:1100@asterisk>;tag=2255010868
To: "1100" <sip:1100@asterisk>
Call-ID: 2707663927@192_168_1_100
CSeq: 918274846 SUBSCRIBE
Contact: <sip:1100@192.168.1.100:5109>
Authorization: Digest username="1100", realm="asterisk", algorithm=MD5, uri="sip:1100@asterisk", nonce="4013b650", response="217a0c7fb8459b1f0d9723e21c472ce6"
Max-Forwards: 70
User-Agent: S450 IP020970000000
Event: message-summary
Expires: 3600
Allow: NOTIFY
Accept: application/simple-message-summary
Content-Length: 0

<------------->
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 0: SUBSCRIBE sip:1100@asterisk SIP/2.0 (35)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 1: Via: SIP/2.0/UDP 192.168.1.100:5109;branch=z9hG4bK3429f3c23251b17ed5f698e1c2f556ec;rport (88)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 2: From: "1100" <sip:1100@asterisk>;tag=2255010868 (47)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 3: To: "1100" <sip:1100@asterisk> (30)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 4: Call-ID: 2707663927@192_168_1_100 (33)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 5: CSeq: 918274846 SUBSCRIBE (25)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 6: Contact: <sip:1100@192.168.1.100:5109> (38)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 7: Authorization: Digest username="1100", realm="asterisk", algorithm=MD5, uri="sip:1100@asterisk", nonce="4013b650", response="217a0c7fb8459b1f0d9723e21c472ce6" (158)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 8: Max-Forwards: 70 (16)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 9: User-Agent: S450 IP020970000000 (31)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 10: Event: message-summary (22)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 11: Expires: 3600 (13)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 12: Allow: NOTIFY (13)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 13: Accept: application/simple-message-summary (42)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 14: Content-Length: 0 (17)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Header 15:  (0)
[Jul  2 12:14:48] VERBOSE[4181] logger.c: --- (15 headers 0 lines) ---
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: = Found Their Call ID: 2707663927@192_168_1_100 Their Tag 2255010868 Our tag: as534c92bc
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: **** Received SUBSCRIBE (10) - Command in SIP SUBSCRIBE
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Got a new subscription 2707663927@192_168_1_100 (possibly with auth)
[Jul  2 12:14:48] VERBOSE[4181] logger.c: Creating new subscription
[Jul  2 12:14:48] VERBOSE[4181] logger.c: Sending to 192.168.1.100 : 5109 (NAT)
[Jul  2 12:14:48] VERBOSE[4181] logger.c: Found peer '1100'
[Jul  2 12:14:48] VERBOSE[4181] logger.c: Looking for 1100 in from-internal (domain asterisk)
[Jul  2 12:14:48] DEBUG[4181] chan_sip.c: Adding subscription for mailbox notification - peer 1100 Mailbox 1100@default
[Jul  2 12:14:48] VERBOSE[4181] logger.c: Scheduling destruction of SIP dialog '2707663927@192_168_1_100' in 3610000 ms (Method: SUBSCRIBE)
[Jul  2 12:14:48] VERBOSE[4181] logger.c: 
<--- Transmitting (no NAT) to 192.168.1.100:5109 --->
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.1.100:5109;branch=z9hG4bK3429f3c23251b17ed5f698e1c2f556ec;rport;received=192.168.1.100
From: "1100" <sip:1100@asterisk>;tag=2255010868
To: "1100" <sip:1100@asterisk>;tag=as534c92bc
Call-ID: 2707663927@192_168_1_100
CSeq: 918274846 SUBSCRIBE
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Expires: 3600
Contact: <sip:1100@192.168.1.16>;expires=3600
Content-Length: 0

################################################## MANUALLY ADDED DESCRIPTION #############################################################
THAT WAS THE SUBSCRIBE PART, NOW LET'S SEE THE OUTBOUND NOTIFY ...
###########################################################################################################################################


---
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: *** SIP TIMER: Initializing retransmit timer on packet: Id  #-1
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Allocating new SIP dialog for (No Call-ID) - NOTIFY (No RTP)
[Jul  2 12:14:47] DEBUG[4181] acl.c: ##### Testing 192.168.1.100 with 192.168.1.0
[Jul  2 12:14:47] VERBOSE[4181] logger.c: Scheduling destruction of SIP dialog '44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16' in 32000 ms (Method: NOTIFY)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 0: NOTIFY sip:1100@192.168.1.100:5109 SIP/2.0 (42)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 1: Via: SIP/2.0/UDP 192.168.1.16:5060;branch=z9hG4bK7d64c85e (57)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 2: From: "Unknown" <sip:Unknown@192.168.1.16>;tag=as4ea75f10 (57)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 3: To: <sip:1100@192.168.1.100:5109> (33)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 4: Contact: <sip:Unknown@192.168.1.16> (35)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 5: Call-ID: 44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16 (54)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 6: CSeq: 102 NOTIFY (16)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 7: User-Agent: Asterisk PBX (24)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 8: Max-Forwards: 70 (16)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 9: Event: message-summary (22)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 10: Content-Type: application/simple-message-summary (48)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 11: Content-Length: 87 (18)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 12:  (0)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Line: Messages-Waiting: no (20)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Line: Message-Account: sip:*97@192.168.1.16 (37)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Line: Voice-Message: 0/0 (0/0) (24)
[Jul  2 12:14:47] VERBOSE[4181] logger.c: Reliably Transmitting (no NAT) to 192.168.1.100:5109:
NOTIFY sip:1100@192.168.1.100:5109 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.16:5060;branch=z9hG4bK7d64c85e
From: "Unknown" <sip:Unknown@192.168.1.16>;tag=as4ea75f10
To: <sip:1100@192.168.1.100:5109>
Contact: <sip:Unknown@192.168.1.16>
Call-ID: 44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16
CSeq: 102 NOTIFY
User-Agent: Asterisk PBX
Max-Forwards: 70
Event: message-summary
Content-Type: application/simple-message-summary
Content-Length: 87

Messages-Waiting: no
Message-Account: sip:*97@192.168.1.16
Voice-Message: 0/0 (0/0)

---


<--- SIP read from 192.168.1.100:5109 --->
SIP/2.0 481 Call Leg/Transaction Does Not Exist
Via: SIP/2.0/UDP 192.168.1.16:5060;branch=z9hG4bK7d64c85e
From: "Unknown" <sip:Unknown@192.168.1.16>;tag=as4ea75f10
To: <sip:1100@192.168.1.100:5109>
Call-ID: 44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16
CSeq: 102 NOTIFY
Content-Length: 0


<------------->
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 0: SIP/2.0 481 Call Leg/Transaction Does Not Exist (47)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 1: Via: SIP/2.0/UDP 192.168.1.16:5060;branch=z9hG4bK7d64c85e (57)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 2: From: "Unknown" <sip:Unknown@192.168.1.16>;tag=as4ea75f10 (57)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 3: To: <sip:1100@192.168.1.100:5109> (33)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 4: Call-ID: 44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16 (54)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 5: CSeq: 102 NOTIFY (16)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 6: Content-Length: 0 (17)
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Header 7:  (0)
[Jul  2 12:14:47] VERBOSE[4181] logger.c: --- (7 headers 0 lines) ---
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: = Found Their Call ID: 44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16 Their Tag  Our tag: as4ea75f10
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: ** SIP TIMER: Cancelling retransmit of packet (reply received) Retransid #177
[Jul  2 12:14:47] DEBUG[4181] chan_sip.c: Stopping retransmission on '44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16' of Request 102: Match Found
[Jul  2 12:14:47] WARNING[4181] chan_sip.c: Remote host can't match request NOTIFY to call '44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16'. Giving up.


############################################## MANUALLY ADDED DESCRIPTION ##################################################
UHH, the Notify is using CALL-ID: 44a2de5123f92fc47f9ff8aa6fcd0e10@192.168.1.16 of which we have no SUBSCRIBE in this trace
But a new Subscribe as occured using CALL-ID: 2707663927@192_168_1_100, why isn't that subscription used and what's causing the domain name to change. 
I see the domainparts "@192.168.1.16" and "@192_168_1_100"? Are two peers interfering with each other??