Archie; The T.38 settings on the Grandstream 4104 don't seem to have much in the way of logic.
T.38 Setting (Syntax: ch x-y: mode=val,rate=val,ecm=val;[...])
1. T.38 Setting:
(mode: 1:Relay(default), 2:Passthough)
(rate: 2400, 4800, 7200, 9600(default), 12000, 14400)
(ecm...