20.2.2.4. Parameter “USE Previous Media Negotiation” has to be set 
If an interworking between MTX4104 connected to SoftGate and MTX4104
connected to CGW takes place the parameter “USE Previous Media Negotiation”
has to be enabled in the MTX.
Figure 430. Scenario with T.38 disabled in the MTX 10.80.212.188
In the Scenario above T.38 is disabled in the MTX 10.80.212.188. Therefore
DMC is allowed. In all other components T.38 is enabled.
Assumed the parameter “USE Previous Media Negotiation” is not
set: First a DMC is established between the both MTX41xx adapters. After
recognizing that a fax is connected the MTX 10.80.212.190 sends a T.38
re-invite, which will be rejected with “488 Not Acceptable” here
by the vHG3500 10.80.212.234. The reason is that it is not possible to
make a renegotiation of features in a DMC interworking scenario on the
CGW. After the reject of T.38 a G.711 re-invite is sent by Mediatrix.
Also this re-invite is rejected due to the reason mentioned above. After
the second reject the MTX41xx drops the call.
If the parameter “USE Previous Media Negotiation” is set, the
MTX41xx continues with G.711 after receiving the “488 Not Acceptable”
for T.38 re-invite and the fax transmission works.