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Have a fax recieving calls on a sip trunk. SIP provider does support fax on SIP.

Fax won't recieve faxes from certain people.

Tested fax to fax via internal extension - all fine

Tested same fax via sip - would not recieve

Trace flaged up

SIP/2.0 488 Not Acceptable Here
To: "anonymous" sip:0143xxxxxxx@83.245.6.81>;tag=3481446663-52583
From: sip:0143xxxxxxx@83.245.6.81:5060>;tag=c7f888d3e8c04bb4
Contact: sip:0143xxxxxxx@83.245.6.81:5060>
Call-ID: 5517760-3481446663-52523@msw3.mydomain.com
CSeq: 2 INVITE
Via: SIP/2.0/UDP 88.215.4.160:5060;rport;branch=z9hG4bK3c14a584e21159a1dcce27440d16f969
Content-Length: 0

What could be wrong & whats the best fix

Who is the SIP provider? What system is this on?

What's in the SDP section of the invite. A 488 is a codec mismatch. Make sure both ends support either PCMU or T.38.

SIP provide is Gamma, system is IPO 500 5.0(18)

Invite message

INVITE sip:0143xxxxxxx@88.215.4.160 SIP/2.0
Max-Forwards: 69
Session-Expires: 3600;Refresher=uac
Supported: timer
To: sip:0143xxxxxxx@83.245.6.81:5060
From: "anonymous" sip:0143xxxxxxx@83.245.6.81 ;tag=3481446663-52583
Contact: sip:0143xxxxxxx@83.245.6.81:5060
Remote-Party-Id: sip:0143xxxxxxx@83.245.6.81 ;party=calling;screen=yes;privacy=full
Call-ID: 5517760-3481446663-52523@msw3.mydomain.com
CSeq: 1 INVITE
Via: SIP/2.0/UDP 83.245.6.81:5060;branch=z9hG4bK78ad2eebc5325c0c196e05601d5c1001
Allow-Events: telephone-event
Content-Type: application/sdp
Content-Length: 257

v=0
o=msw 2347 712 IN IP4 83.245.6.81
s=sip call
c=IN IP4 83.245.6.82
t=0 0
m=audio 12006 RTP/AVP 8 0 100 101
a=fmtp:101 0-16
a=rtpmap:101 telephone-event/8000
a=fmtp:100 192-194
a=rtpmap:100 X-NSE/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000