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New Member

Issues with SIP trunk receive fax's on 2921

Hi All

We are having issues with receive fax's over our SIP trunk.  Voice calls ok no issues at all. the fax's are not using T38.   The problem we have is that with some fax's we only receive 1/2 a page, others nothing at all, sender and receive get comm error and some that are received ok.  We have found that some fax machines work better than others, but even the best fax we have so far fails after about 3 pages of receive with COMM error.  Most times if you send a test  fax with just "test fax" on paper its received ok, however if you send a fax with images etc only 1/2 page is received.  Most people seem to say this is common issue.  Is there some standard settings that should be used ? Have tried with modem passthrough and without, tried ctraete in/out dial pear with various settings but no luck so far.  Any idea what we should set and what should not be set to get this to work. 


Hall of Fame Super Silver

Issues with SIP trunk receive fax's on 2921

Is T38 not available with the provider?  That is the best option.

Pass-through is OK in most cases as long as it's consistent end to end.  What device is the fax connected to?

Can you post your config?


New Member

Issues with SIP trunk receive fax's on 2921

We have a mixture of ATA186 and 187's, hense no T38.  Will get a copy of config up as soon as possible.

Issues with SIP trunk receive fax's on 2921

Couple of vague suggestions:

- Check, double check & tripple check your QoS settings across the board. Faxes are very sentitive to packet delay & loss. Look at the CUCM CMR records to see what the call stats are like of these failed fax calls.

- Make sure you're using the same codec end-to-end. Transcoding can add delays and unnecessary complexity.

- Try and avoid using MTPs if you can.


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