You are going to need some sort of an interface.
If its on an extension port, you need to showw 600 ohm on the line, to simulate a phone. This can be done with a Bogen WMT1-a. Once connected, reboot the system.
If you are using a trunk port, you need to have loop voltage on teh line or the IPO will not let the page code go out on the trunk. In this case use a UPAM.
But, no matter what way you do it, the amp can not provide loop current or the 600ohm, so you need an interface. I think I have heard of guys using the OPT port on the back..but never seen it.