Не идут звонки по IAX

Есть 2 сервера Asterisk 1.6.2.24

Сервер А [iax.conf]:

[kazan]
type=friend
host=XXX.XXX.XXX.XXX
auth=MD5
trunk=yes
secret=welcome
context=kazan

Сервер Б [iax.conf]:

[volgograd]
type=friend
auth=MD5
host=XXX.XXX.XXX.XXX
trunk=yes
secret=welcome
context=volgograd

Звонок с Сервера А на Сервер Б проходит нормально, а вот обратно с Сервера Б на сервер А вообще не идет. В чем может быть причина?

Логи на Сервер А [iax debug]:

Rx-Frame Retry[ No] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW
   Timestamp: 00014ms  SCall: 00977  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   VERSION         : 2
   CALLED NUMBER   : 4777
   CODEC_PREFS     : (gsm)
   CALLING NUMBER  : 8432XXXXXXX
   CALLING PRESNTN : 3
   CALLING TYPEOFN : 33
   CALLING TRANSIT : 0
   CALLING NAME    :
   LANGUAGE        : en
   USERNAME        : kazan
   FORMAT          : 2
   CAPABILITY      : 2
   ADSICPE         : 2
   DATE TIME       : 2012-10-26  14:51:48

Rx-Frame Retry[Yes] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW
   Timestamp: 00014ms  SCall: 00977  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   VERSION         : 2
   CALLED NUMBER   : 4777
   CODEC_PREFS     : (gsm)
   CALLING NUMBER  : 8432XXXXXXX
   CALLING PRESNTN : 3
   CALLING TYPEOFN : 33
   CALLING TRANSIT : 0
   CALLING NAME    :
   LANGUAGE        : en
   USERNAME        : kazan
   FORMAT          : 2
   CAPABILITY      : 2
   ADSICPE         : 2
   DATE TIME       : 2012-10-26  14:51:48

Rx-Frame Retry[ No] -- OSeqno: 001 ISeqno: 000 Type: IAX     Subclass: HANGUP
   Timestamp: 04015ms  SCall: 00977  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   CAUSE CODE      : 0

Rx-Frame Retry[Yes] -- OSeqno: 001 ISeqno: 000 Type: IAX     Subclass: HANGUP
   Timestamp: 04015ms  SCall: 00977  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   CAUSE CODE      : 0

Rx-Frame Retry[Yes] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW
   Timestamp: 00014ms  SCall: 00977  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   VERSION         : 2
   CALLED NUMBER   : 4777
   CODEC_PREFS     : (gsm)
   CALLING NUMBER  : 8432XXXXXXX
   CALLING PRESNTN : 3
   CALLING TYPEOFN : 33
   CALLING TRANSIT : 0
   CALLING NAME    :
   LANGUAGE        : en
   USERNAME        : kazan
   FORMAT          : 2
   CAPABILITY      : 2
   ADSICPE         : 2
   DATE TIME       : 2012-10-26  14:51:48

Сервер Б [iax debug]:

Tx-Frame Retry[000] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW
   Timestamp: 00014ms  SCall: 03512  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   VERSION         : 2
   CALLED NUMBER   : 4777
   CODEC_PREFS     : (gsm)
   CALLING NUMBER  : 8432XXXXXXX
   CALLING PRESNTN : 3
   CALLING TYPEOFN : 33
   CALLING TRANSIT : 0
   CALLING NAME    :
   LANGUAGE        : en
   USERNAME        : kazan
   FORMAT          : 2
   CAPABILITY      : 2
   ADSICPE         : 2
   DATE TIME       : 2012-10-26  14:54:16

Rx-Frame Retry[Yes] -- OSeqno: 000 ISeqno: 001 Type: IAX     Subclass: CTOKEN
   Timestamp: 00014ms  SCall: 00001  DCall: 03512 [XXX.XXX.XXX.XXX:4569]
   Unknown IE 054  : Present

Tx-Frame Retry[001] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW
   Timestamp: 00014ms  SCall: 03512  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   VERSION         : 2
   CALLED NUMBER   : 4777
   CODEC_PREFS     : (gsm)
   CALLING NUMBER  : 8432XXXXXXX
   CALLING PRESNTN : 3
   CALLING TYPEOFN : 33
   CALLING TRANSIT : 0
   CALLING NAME    :
   LANGUAGE        : en
   USERNAME        : kazan
   FORMAT          : 2
   CAPABILITY      : 2
   ADSICPE         : 2
   DATE TIME       : 2012-10-26  14:54:16

Rx-Frame Retry[Yes] -- OSeqno: 000 ISeqno: 001 Type: IAX     Subclass: CTOKEN
   Timestamp: 00014ms  SCall: 00001  DCall: 03512 [XXX.XXX.XXX.XXX:4569]
   Unknown IE 054  : Present

Tx-Frame Retry[000] -- OSeqno: 001 ISeqno: 000 Type: IAX     Subclass: HANGUP
   Timestamp: 04015ms  SCall: 03512  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   CAUSE CODE      : 0

Tx-Frame Retry[001] -- OSeqno: 001 ISeqno: 000 Type: IAX     Subclass: HANGUP
   Timestamp: 04015ms  SCall: 03512  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   CAUSE CODE      : 0

Tx-Frame Retry[002] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW
   Timestamp: 00014ms  SCall: 03512  DCall: 00000 [XXX.XXX.XXX.XXX:4569]
   VERSION         : 2
   CALLED NUMBER   : 4777
   CODEC_PREFS     : (gsm)
   CALLING NUMBER  : 8432XXXXXXX
   CALLING PRESNTN : 3
   CALLING TYPEOFN : 33
   CALLING TRANSIT : 0
   CALLING NAME    :
   LANGUAGE        : en
   USERNAME        : kazan
   FORMAT          : 2
   CAPABILITY      : 2
   ADSICPE         : 2
   DATE TIME       : 2012-10-26  14:54:16

Rx-Frame Retry[Yes] -- OSeqno: 000 ISeqno: 001 Type: IAX     Subclass: CTOKEN
   Timestamp: 00014ms  SCall: 00001  DCall: 03512 [XXX.XXX.XXX.XXX:4569]
   Unknown IE 054  : Present



   -- Accepting call from '843XXXXXXX' to '4777' on channel 0/23, span 1
    -- Executing [4777@from_ats:1] Dial("DAHDI/23-1", "IAX2/kazan:welcome@XXX.XXX.XXX.XXX:4569/4777,15,t") in new stack
    -- Called kazan:welcome@XXX.XXX.XXX.XXX:4569/4777
[Oct 26 14:54:21] NOTICE[1664]: chan_iax2.c:4435 __auto_congest: Auto-congesting call due to slow response
    -- IAX2/10.0.0.11:4569-3512 is circuit-busy
    -- Hungup 'IAX2/XXX.XXX.XXX.XXX:4569-3512'
  == Everyone is busy/congested at this time (1:0/1/0)
    -- Executing [4777@from_ats:2] Hangup("DAHDI/23-1", "") in new stack
  == Spawn extension (from_ats, 4777, 2) exited non-zero on 'DAHDI/23-1'
    -- Hungup 'DAHDI/23-1'