Fax relay does not require any H323 capability exchange or H323 signaling.
What happens is, H323 will be used to setup the call and after the call is setup, the DSP's in the gateway would listen for FAX tones. If they detect any FAX tone, then FAX Relay would kick in and do the necessary FAX signaling conversion and other required things.
Now, when you say Callmanager is in between the two gateways, do you mean the Callmanager is controlling the gateways? If that is the case, then the only difference is that the gateways would be using MGCP for call setup, and if they are configured for Cisco FAX RELAY, then after the FAX TONES are detected,they would move on to FAX RELAY mode.
Regards,
Anup
