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Схема: ТА----ap100----ip--SIP-server
Обрываются по busy входящие звонки.
Дебаг след:

5 <Call 11> : Connected from(0)
6 <SIP 11> : SetConnected
7 <SIP 11> : Add Local Audio MediaFormat : 18

Sending SIP PDU to ( 78.31.23.253:5060 ) from 5060
SIP/2.0 200 OK
Via: SIP/2.0/UDP 78.31.23.253:5060;branch=z9hG4bKsv6kgu105os17lo8b5s0.1
From: "Vladimir Shishkov"<sip:84956621322@195.161.10.21;user=phone>;tag=82203252
1-1269352268228-
To: "gigacoms101 gigacoms101"<sip:gigacoms101@new-tc.ru>;tag=2d006b12a4
Call-ID: BW165108228230310-2128979847@195.161.10.21
CSeq: 94716131 INVITE
Supported: timer, replaces, early-session
User-Agent: AddPac SIP Gateway
Contact: sip:gigacoms101@172.31.111.99
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, UPDATE, REFER, NOTIFY, INFO
Content-Type: application/sdp
Content-Length: 179

v=0
o=gigacoms101 1586 1586 IN IP4 172.31.111.99
s=AddPac Gateway SDP
c=IN IP4 172.31.111.99
t=0 0
m=audio 23008 RTP/AVP 18
a=ptime:20
a=rtpmap:18 G729/8000/1
a=sendrecv


Received SIP PDU from ( 78.31.23.253:5060 )
ACK sip:gigacoms101@172.31.111.99:5060 SIP/2.0
Via: SIP/2.0/UDP 78.31.23.253:5060;branch=z9hG4bK5999fe108g70ih0ja7k0.1
From: "Vladimir Shishkov"<sip:84956621322@195.161.10.21;user=phone>;tag=82203252
1-1269352268228-
To: "gigacoms101 gigacoms101"<sip:gigacoms101@new-tc.ru>;tag=2d006b12a4
Call-ID: BW165108228230310-2128979847@195.161.10.21
CSeq: 94716131 ACK
Contact: <sip:84956621322@78.31.23.253:5060;transport=udp>
Max-Forwards: 9
Content-Length: 0


8 <SIP 11> : ACK received
9 <SIP 11> : Receive ACK Request
10 <SIP 11> : Set Terminated Success for 94716131 INVITE
11 <CEP 000000> : Disconnected(16) at Busy
12 <Call 11> : Terminated from(0) this(Local:CallClear) before(NULL)
forced(0) time(1586)
13 <SIP 11> : ReleaseWithBYE
14 <SIP 11> : Send BYE Request

Sending SIP PDU to ( 78.31.23.253:5060 ) from 5060
BYE sip:84956621322@78.31.23.253;transport=udp SIP/2.0
Via: SIP/2.0/UDP 172.31.111.99:5060;branch=z9hG4bK2d006b12a44
From: "gigacoms101 gigacoms101"<sip:gigacoms101@new-tc.ru>;tag=2d006b12a4
To: "Vladimir Shishkov"<sip:84956621322@195.161.10.21;user=phone>;tag=822032521-
1269352268228-
Call-ID: BW165108228230310-2128979847@195.161.10.21
CSeq: 4 BYE
Date: Thu, 01 Jan 1970 00:26:26 GMT
User-Agent: AddPac SIP Gateway
Contact: <sip:gigacoms101@172.31.111.99>
Content-Length: 0
Max-Forwards: 70


Что можно подкрутить, конфиг следующий:
Current configuration:
!
version 8.41.017
!
hostname AP100
!
!
!
interface Loopback0
ip address 127.0.0.1 255.0.0.0
!
interface FastEthernet0/0
ip address 172.31.111.99 255.255.0.0
ip nat outside
speed auto
no qos-control
!
interface FastEthernet0/1
ip address 192.168.10.1 255.255.255.0
ip nat inside
speed auto
no qos-control
!
ip route 0.0.0.0 0.0.0.0 172.31.0.253 10
!
access-list 100 permit ip 192.168.10.0 0.0.0.255 any
!
!
ip nat inside source list 100 interface FastEthernet0/0 overload
!
!
snmp name AP100_G2
!
!
http server
!
!
dns name-server 172.31.0.253
!
!
!
!
! VoIP configuration.
!
!
! Voice service voip configuration.
!
voice service voip
fax protocol t38 redundancy 0
fax rate 9600
h323 call start fast
h323 call tunnel enable
busyout monitor sip-server
!
!
! Voice port configuration.
!
! FXS
voice-port 0/0
caller-id enable
!
!
!
!
! Pots peer configuration.
!
dial-peer voice 0 pots
destination-pattern T
port 0/0
user-name ЫЫЫ
user-password ЫЫЫ
!
!
!
! Voip peer configuration.
!
dial-peer voice 1000 voip
destination-pattern T
session target sip-server
session protocol sip
voice-class codec 0
no dtmf-relay
no vad
!
!
!
!
!
!
! Gateway configuration.
!
gateway
no ignore-msg-from-other-gk
!
!
! Codec classes configuration.
!
voice class codec 0
codec preference 1 g729
codec preference 2 g711alaw
codec preference 3 g7231r63
codec preference 4 g711ulaw
!
!
!
! SIP UA configuration.
!
sip-ua
user-register
sip-username ЫЫЫ
sip-password ЫЫЫ
sip-server 78.31.23.253
srv enable
media-channel early
set-local-domain new-tc.ru
register e164
!
!
! MGCP configuration.
!
mgcp
codec g711ulaw
vad
!
!
! Tones
!
!
!
!
line console
!
line vty
!
end

В чем проблема?