It would be possible to make this work but it is a little difficult because there will be some programming needed to be done. I would call Tech Support at 715-386-8666 and speak with a Technician and explain your situation.
Sorry about the delay I forget to check the Forum for questions, usually the questions come in on the Helpdesk tickets. What you will need to do if you have programmed *5 you will need to get it back to factory default described on page 7 of the manual Accessing the Programming Mode Locally (When *5 has been programmed. When you get it back to default you will connect your phone line to "Doorbox 1, your phone to Line Out to Phones, and turn the "Talk Battery" switch to ON. You will have to press #1 to make a phone call to get Dial Tone but at the same time you could press ** to activate the relay.