Tried it several times. I can't seem to get it to work. Might because I have an old fax machine but I don't know. The call seems to go into DR and not the fax gateway.
I have not spent a lot of time investigating it though.
I have a grandstream 4104 and it doesnt work? in the manual theres no info, this is something that a client was very interested and it was a selling point and its not working, the only way out for fax to work its having a separated line out of 3cx only for fax, but thats not something that they want.