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New Member

T.38 fax behind a NAT

We are trying to use some T.38 fax software (non-Cisco) from behind an 800-series router (877). Inbound faxes work fine but outbound ones fail every time, the software claims that the remote machine never responds.

Is there any special configuration necessary to get this to work on a basic router, considering the T.38 is going through a NAT?

The 877 is aware of SIP/RTP but I'm not sure if it can pass T.38 properly as I believe it uses UDP-TL instead of RTP? If the UDP-TL stream is not being negotiated properly this would seem to explain the "no response" error if the channel doesn't every get set up properly.

I should add also that SIP voice calls through the 877 are working fine, so it is allowing some SIP communications to work.


Re: T.38 fax behind a NAT

T38 should not be an issue through NAT. Only protocols that embed IP addresses are subject to NAT issues (SIP, SCCP, etc).

T38 only has data, and is not susceptible to NAT. SIP may, but not T38.


New Member

Re: T.38 fax behind a NAT

Thanks very much for the confirmation. We actually set up the router temporarily to route directly just a few hours ago and T.38 is having the exact same issue, so it definately isn't a NAT issue.