Unfortunately, that is going to happen, it's designed to be used with an answering machine or a single line phone.. The Brother won't "hear" an incoming fax, and then cut off the line, until the phone line is answered by the gateway. What you can do is not use that phone number for incoming voice calls. Set the gateway so that that line is a separate trunk group, then set the gateway to never send that call onto 3CX. If the new trunk is added as a second choice, outbound route in 3CX it will still be there, for overflow, if all other trunks are busy.