I have two voice gateways (2811 and VG224), both configured with MGCP. I am not using the "ccm-manager config server" command, to stop CM from configuring the gateway.
I placed a fax call between both gateways, controlled with a CM 5.1(2)b. The fax call completes successful everytime and under "show voice call sum" it indicates that the fax protocol is T.38.
Gateway to Gateway MGCP T.38 is supported... according to this link: