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Intermittant Fax Issues with Gateways using SIP Trunks


Client is having intermittant issues with g711 pass-through enabled gateways, both a 2951 and a couple 2851 gateways.

Faxes are on VG 224's dialing out over a CUCM sip trunk to the gateway which has a conventional PRI. SIP-H323 dial-peers.

Some of the issues are to outside fax machines, and soime are to internal fax machines at other locations where faxing by 5 digits succeeds and faxing to same machine using 10 digit local pstn seems to fail the majority of the time.

Config snipnet follows from routers, Vg 224 is mgcp connected to CUCM.

voice service voip


media statistics

allow-connections h323 to h323

allow-connections h323 to sip

allow-connections sip to h323

allow-connections sip to sip

redirect ip2ip

fax protocol pass-through g711ulaw



  early-offer forced

  midcall-signaling passthru


voice class codec 1

codec preference 1 g711ulaw

codec preference 2 g729r8

dial-peer voice 1 pots

description Local 10 digits

preference 1

destination-pattern 9[2-9]..[2-9]......


port 0/0/0:23

forward-digits 10

dial-peer voice 9 pots

description ** Inbound Calls **

translation-profile incoming XlateAllInbound

incoming called-number .


port 0/0/0:23

dial-peer voice 200 voip

description dial-peer to PUB

preference 2

destination-pattern 86...

session protocol sipv2

session target

incoming called-number .

voice-class codec 1 

dtmf-relay rtp-nte

fax rate disable

ip qos dscp cs3 signaling

no vad

I was thinkign of adding under voice service voip/h323/emptcapability but not sure if that is needed.

Thanks for any insight as to what the issue may be, again dialing over the network using 5 digits seems to always work, but dialing pstn with 10 digit local seems to fail, calls are indeed local calls, just down the street to another building, tried dialing it as LD with 91 and calls seems to work as LD. Calls are going out same PRI whether 10 digit local or LD, These faxes have been working for a long time, and recently started having trouble which may or may not coincide with changing the gateways from MGCP over to a CUCM SIP Trunk.



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Intermittant Fax Issues with Gateways using SIP Trunks

With pass-through, there is no guarantee that fax works over IP.

You should check with you SIP provider how they support fax relay, and make it work.

Intermittant Fax Issues with Gateways using SIP Trunks

The only guaranteed way you can get a good success ratio on FAX over a SIP trunk is with T38 delivery. Checked with your carrier if they support T38, if not I would get some analog lines for the FAX connections. Pass-through is not guaranteed.

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