Thanks. Here's the SIP trace:
== Using SIP RTP CoS mark 5
-- Executing [2000@internal:1] AGI("SIP/6005-000004af", "sarkhpe,Alias,SIP/6000 Local/07xxxxxxxxx@internal,2000,") in new stack
-- Launched AGI Script /var/lib/asterisk/agi-bin/sarkhpe
-- AGI Script Executing Application: (Set) Options: (CALLERID(rdnis)=2000)
-- AGI Script Executing Application: (Dial) Options: (SIP/6000&Local/07xxxxxxxxx@internal,150,ctI)
== Using SIP RTP CoS mark 5
-- Called SIP/6000
-- Called Local/07xxxxxxxxx@internal
-- Connected line update to SIP/6005-000004af prevented.
-- Executing [07xxxxxxxxx@internal:1] AGI("Local/07xxxxxxxxx@internal-5983;2", "sarkhpe,OutRoute,Mobile,,") in new stack
-- Launched AGI Script /var/lib/asterisk/agi-bin/sarkhpe
-- Local/07xxxxxxxxx@internal-5983;1 answered SIP/6005-000004af
-- AGI Script Executing Application: (Set) Options: (TIMEOUT(absolute)=14400)
Channel will hangup at 2013-05-29 17:39:27.280 BST.
-- AGI Script Executing Application: (Dial) Options: (SIP/07xxxxxxxxx@peer2150,,T)
== Using SIP RTP CoS mark 5
Audio is at 5060
Adding codec 0x100 (g729) to SDP
Adding codec 0x8 (alaw) to SDP
Adding codec 0x4 (ulaw) to SDP
Adding non-codec 0x1 (telephone-event) to SDP
Reliably Transmitting (no NAT) to **SIP provider IP**:5060:
INVITE sip:07xxxxxxxxx@**SIP provider domain name** SIP/2.0
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
Max-Forwards: 70
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>
Contact: <sip:**ISDN number**@**PBX Ext IP**:5060>
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 INVITE
User-Agent: Asterisk PBX 1.8.7.0
Date: Wed, 29 May 2013 12:39:27 GMT
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY, INFO, PUBLISH
Supported: replaces, timer
Diversion: <sip:2000@**PBX Ext IP**>;reason=unknown
Content-Type: application/sdp
Content-Length: 309
v=0
o=root 1365188724 1365188724 IN IP4 **PBX Ext IP**
s=Asterisk PBX 1.8.7.0
c=IN IP4 **PBX Ext IP**
t=0 0
m=audio 15940 RTP/AVP 18 8 0 101
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:8 PCMA/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=ptime:20
a=sendrecv
---
-- Called SIP/07xxxxxxxxx@peer2150
<--- SIP read from UDP:**SIP provider IP**:5060 --->
SIP/2.0 100 Giving a try
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 INVITE
Server: OpenSIPS (1.6.4-2-notls (x86_64/linux))
Content-Length: 0
<------------->
--- (8 headers 0 lines) ---
Reliably Transmitting (no NAT) to **SIP provider IP**:5060:
OPTIONS sip:**SIP provider domain name** SIP/2.0
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK4d335944
Max-Forwards: 70
From: "asterisk" <sip:asterisk@**PBX Ext IP**>;tag=as2731a4ce
To: <sip:**SIP provider domain name**>
Contact: <sip:asterisk@**PBX Ext IP**:5060>
Call-ID: 433e18fb50afef3c6f46649731d814da@**PBX Ext IP**:5060
CSeq: 102 OPTIONS
User-Agent: Asterisk PBX 1.8.7.0
Date: Wed, 29 May 2013 12:39:37 GMT
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY, INFO, PUBLISH
Supported: replaces, timer
Content-Length: 0
---
<--- SIP read from UDP:**SIP provider IP**:5060 --->
SIP/2.0 500 Method/domain not supported on SIP Trunk Out
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK4d335944
From: "asterisk" <sip:asterisk@**PBX Ext IP**>;tag=as2731a4ce
To: <sip:**SIP provider domain name**>;tag=7377d6fcaaaac332b0d1d095d1ff234a.6d0c
Call-ID: 433e18fb50afef3c6f46649731d814da@**PBX Ext IP**:5060
CSeq: 102 OPTIONS
Server: OpenSIPS (1.6.4-2-notls (x86_64/linux))
Content-Length: 0
<------------->
--- (8 headers 0 lines) ---
Really destroying SIP dialog '433e18fb50afef3c6f46649731d814da@**PBX Ext IP**:5060' Method: OPTIONS
<--- SIP read from UDP:**SIP provider IP**:5060 --->
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>;tag=as6ff9a8c8
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Contact: <sip:*77136147%2307xxxxxxxxx@212.11.91.73>
Content-Length: 0
<------------->
--- (10 headers 0 lines) ---
-- SIP/peer2150-000004b1 is ringing
-- Local/07xxxxxxxxx@internal-5983;2 requested special control 20, passing it to SIP/peer2150-000004b1
-- Local/07xxxxxxxxx@internal-5983;2 requested special control 20, passing it to SIP/peer2150-000004b1
<--- SIP read from UDP:**SIP provider IP**:5060 --->
SIP/2.0 183 Session Progress
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>;tag=as6ff9a8c8
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Contact: <sip:*77136147%2307xxxxxxxxx@212.11.91.73>
Content-Type: application/sdp
Content-Length: 240
v=0
o=root 12059 12059 IN IP4 212.11.91.73
s=session
c=IN IP4 212.11.91.73
t=0 0
m=audio 14766 RTP/AVP 8 101
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=silenceSupp:off - - - -
a=ptime:20
a=sendrecv
<------------->
--- (11 headers 12 lines) ---
Found RTP audio format 8
Found RTP audio format 101
Found audio description format PCMA for ID 8
Found audio description format telephone-event for ID 101
Capabilities: us - 0x10c (ulaw|alaw|g729), peer - audio=0x8 (alaw)/video=0x0 (nothing)/text=0x0 (nothing), combined - 0x8 (alaw)
Non-codec capabilities (dtmf): us - 0x1 (telephone-event|), peer - 0x1 (telephone-event|), combined - 0x1 (telephone-event|)
Peer audio RTP is at port 212.11.91.73:14766
-- SIP/peer2150-000004b1 is making progress passing it to Local/07xxxxxxxxx@internal-5983;2
-- Local/07xxxxxxxxx@internal-5983;2 requested special control 20, passing it to SIP/peer2150-000004b1
-- Local/07xxxxxxxxx@internal-5983;2 requested special control 20, passing it to SIP/peer2150-000004b1
-- Executing [h@internal:1] Hangup("SIP/6005-000004af", "") in new stack
== Spawn extension (internal, h, 1) exited non-zero on 'SIP/6005-000004af'
-- Local/07xxxxxxxxx@internal-5983;2 requested special control 20, passing it to SIP/peer2150-000004b1
Scheduling destruction of SIP dialog '5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060' in 6400 ms (Method: INVITE)
Reliably Transmitting (no NAT) to **SIP provider IP**:5060:
CANCEL sip:07xxxxxxxxx@**SIP provider domain name** SIP/2.0
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
Max-Forwards: 70
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 CANCEL
User-Agent: Asterisk PBX 1.8.7.0
Reason: SIP;cause=200;text="Call completed elsewhere"
Content-Length: 0
---
Scheduling destruction of SIP dialog '5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060' in 6400 ms (Method: INVITE)
-- <SIP/6005-000004af>AGI Script sarkhpe completed, returning 4
== Spawn extension (internal, 2000, 1) exited non-zero on 'SIP/6005-000004af'
-- <Local/07xxxxxxxxx@internal-5983;2>AGI Script sarkhpe completed, returning 4
== Spawn extension (internal, 07xxxxxxxxx, 1) exited non-zero on 'Local/07xxxxxxxxx@internal-5983;2'
-- Executing [h@internal:1] Hangup("Local/07xxxxxxxxx@internal-5983;2", "") in new stack
== Spawn extension (internal, h, 1) exited non-zero on 'Local/07xxxxxxxxx@internal-5983;2'
<--- SIP read from UDP:**SIP provider IP**:5060 --->
SIP/2.0 200 canceling
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>;tag=e679694d172e7ed7ac82cbbb8045e301-fc13
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 CANCEL
Server: OpenSIPS (1.6.4-2-notls (x86_64/linux))
Content-Length: 0
<------------->
--- (8 headers 0 lines) ---
<--- SIP read from UDP:**SIP provider IP**:5060 --->
SIP/2.0 487 Request Terminated
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>;tag=as6ff9a8c8
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Content-Length: 0
<------------->
--- (9 headers 0 lines) ---
Transmitting (no NAT) to **SIP provider IP**:5060:
ACK sip:07xxxxxxxxx@**SIP provider domain name** SIP/2.0
Via: SIP/2.0/UDP **PBX Ext IP**:5060;branch=z9hG4bK5c3d7df1
Max-Forwards: 70
From: "**ISDN number**" <sip:**ISDN number**@**PBX Ext IP**>;tag=as3f8f18b3
To: <sip:07xxxxxxxxx@**SIP provider domain name**>;tag=as6ff9a8c8
Contact: <sip:**ISDN number**@**PBX Ext IP**:5060>
Call-ID: 5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060
CSeq: 102 ACK
User-Agent: Asterisk PBX 1.8.7.0
Content-Length: 0
---
Really destroying SIP dialog '5ee1ad47173ff0b7515c664041b196a1@**PBX Ext IP**:5060' Method: INVITE