Странная какая-то ерунда....
оказывается после какого-то периода станция шлет пакеты на регистрацию в каком-то урезанном виде, на что в ответ ее и не регистрируют...
Нормальный пакет 623 байта к примеру, а урезанный 438

Кусок нормального:
Internet Protocol, Src: 88.81.238.126 (88.81.238.126), Dst: 77.88.204.2 (77.88.204.2)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 605
Identification: 0x4550 (17744)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 254
Protocol: UDP (0x11)
Header checksum: 0xd514 [correct]
[Good: True]
[Bad : False]
Source: 88.81.238.126 (88.81.238.126)
Destination: 77.88.204.2 (77.88.204.2)
User Datagram Protocol, Src Port: sip (5060), Dst Port: sip (5060)
Source port: sip (5060)
Destination port: sip (5060)
Length: 585
Checksum: 0xebd5 [correct]
[Good Checksum: True]
[Bad Checksum: False]
Session Initiation Protocol
Request-Line: REGISTER sip:77.88.204.2 SIP/2.0
Method: REGISTER
[Resent Packet: True]
[Suspected resend of frame: 23094]
такой же кусок битого:
Internet Protocol, Src: 88.81.238.126 (88.81.238.126), Dst: 77.88.204.2 (77.88.204.2)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 420
Identification: 0x510d (20749)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 254
Protocol: UDP (0x11)
Header checksum: 0xca10 [correct]
[Good: True]
[Bad : False]
Source: 88.81.238.126 (88.81.238.126)
Destination: 77.88.204.2 (77.88.204.2)
User Datagram Protocol, Src Port: sip (5060), Dst Port: sip (5060)
Source port: sip (5060)
Destination port: sip (5060)
Length: 400
Checksum: 0xa3cb [correct]
[Good Checksum: True]
[Bad Checksum: False]
Session Initiation Protocol
Request-Line: REGISTER sip:77.88.204.2 SIP/2.0
Method: REGISTER
[Resent Packet: False]