20.2.2.3. DMC not established with MTX41xx 
If the sending fax is connected to an MTX41xx adapter in a DMC scenario
and T.38 is enabled the DMC is often rejected.
The MTX41xx switches in the default configuration to T.38 after detecting
a CNG tone. If the sending fax machine is connected to the MTX41xx adapter
the possibility is very high, that the first CNG tone is detected before
the receiving fax machines goes off-hook and the DMC is established.
This means in this case the master channel between MTX41xx and SoftGate
is reconfigured to T.38 before the DMC request is received and therefore
the DMC is rejected.
Workaround In the MTX41xx there is the parameter defaultMachineDetectionCngToneDetection.
If this parameter is set to disable, then the MTX41xx switches to T.38
only after detecting V.21 flags.
Remarks
- If the CNG tone detection is disabled the switch to T.38 takes place later. On the other hand the Mediatrix adapter detects the V.21 flags only on the SCN side. This means the adapter on the sending side is detecting the fax very late (Commerzbank problem) and it is important that the fax is detected on the receive side.
This problem can only occur if the MTX41xx is connected to the SoftGate.
- The parameter “USE Previous Media Negotiation” exists only in the DGWv2.0 load ware from Mediatrix. Therefore an upgrade from the SIPv5.0 to the DGWv2.0 load ware has to be performed on the MTX41xx devices. The tests are made with the DGWv2.0 load ware at the moment.