I just bought aRJ 31X phone jack which is supposed to allow my father in laws life line system to dial out even if his phone is off the hook. I need help hooking this into the existing phone line.

I am not familiar with that jack. If it has terminals, chances are all you ned to do is connect the red wire to the R and the green wire to the G. Post back with details on the jack and where you are wanting to connect it.

This phone jack has 8 terminals The wiring diagram shows a wire jumping between 1 & 4 and another between 5 & 8. 4 is red and 5 isgreen.I assume that I would do that when I hook it up. I would like to just replace the existing 4 wire jack with this Rj31X jack. The phone plugs into the life line unitand the unit plugs into the jack the unit must have something in it that senses if the phone is off the hook and by passes that line and uses the other set of wires. What do you think ?

All the phone stuff is set up with 4 terminals, although only 2 are used with a single line. Does the life line have a modular plug, and the other 2 lines are hardwired? My guess is that the jack is set up to be the first one connected to the incoming phone line. If the life line is activated, it cuts off the ongoing lines at least if they are off the hook. I am guessing 1 and 5 are the incoming red and green lines and 4 and 8 are for the other phones to connect to. Does 1 and 5 connect to the contacts the lifeline phone plug red and green wires connect to? Trouble is, I am not sure you should use the lifeline to check it. You may have to go back to where you bought the jack for more info.

Old Post, but the answer is important.
Red and Green from Telco go to red and green on RJ 31X Gray and brown go to premise wiring(inside phone wiring. In Alarms systems you plug in a 31 X cord to Alarm Panel. If an Alarm condition exis, the Panel lifts the premise phones(Inside phones), so it will not stop panel from dialing.
Another meathod is to install a Silicone Bilateral switch(a little diode) that allows a phone or phones priority. When it goes off hook, all other extensions will be dead(will not stop from dialing out).