Most Popular
Recently Added
Recently Updated

Support for the T.38 Protocol

ThinkTel supports T.38 in the following scenarios:

2-way Direct media VoIP calls.
2-way TDM-to-VoIP (and vice versa) calls.
2-way TDM-to-TDM calls between two different Universal Media Gateways (UMG).

T.38 is not supported in the following scenarios:

Non-direct media VoIP calls, regardless of whether T.38 is supported by the endpoints.
Multi-way calls (including tapped calls, for instance).
Faxes using V.34
Calls where an Originating or Terminating Application Server has been invoked.
BLES-to-VoIP (and vice versa) calls.
Calls where no leg is over IP (!)


A list of the rate centers that DO NOT support T.38 or faxing can be found in uControl. Via Support tab --> Local Number Portability --> Enter 10 digit numbers or NPA-NXX Prefixes in the search field --> Click 'Submit'.
You will be redirected to another page, far column is Network Type. If 'TDM' T.38 is supported, if 'SIP' T.38 is not supported.

ThinkTel has a workaround in place for numbers that are network type SIP. When a fax is sent to/from a number that doesn't support T.38 or G711, we will send the call out our fax loopback trunk and this will complete as TDM and is now supported in all scenarios as outlined above.

If you have a SIP number that you intend to use for faxing you can add to the loopback list by flagging as a fax line in uControl. Here are the instructions Fax - Adding a DID to fax loopback trunk

Behavior that initiates T.38:

  1. If a fax-specific tone is detected, the call will be reprogrammed to G.711 (if it is not already using it).
  2. If a reINVITE was received from a SIP device, it will be rejected. This should not cause the call to fail - instead the SIP device should send a reINVITE requesting the use of G.711 instead

Note that some tones are used in both fax and modem calls. If such a tone is detected on a call using a low-bandwidth codec, the CFS will reprogram the call to G.711. If a fax-specific tone is subsequently detected, the call will only then be reprogrammed to use T.38. This "double fallback" is actually the normal case, since fax calls tend to start with the CNG or CED tones, and these are also used in modem calls.

Properties ID: 000055   Views: 11666   Updated: 5 years ago
Filed under:
Twitter - ThinkTel Linkedin - ThinkTel Youtube - ThinkTel