You are dealing with different devices. In that case of "other" VoIP" (Android) devices, it may be that a transfer function is not supported because the application was not intended to be used with anything more than "basic" call functions.
In the case of the Grandstream...how was the set provisioned? Is it "officially" supported, or was a provisioning template supplied by GS used?
DTMF feature functions are usually proceeded with a flash, as i said. That, of course, cant be done on an App, or a VoIP set.
I would suggest using (at least trying) the 3CX Android App on the Android set, and, perhaps contacting Grandstream about he transfer function on the set in question, not working.