There is a opx available for the panther systems. and it is fine for the likes of a answering machine, cordless, or long line.
this is a converter that plugs in to key port and gives out a slt port, it interfaces with the system as a key phone. the goldstar GSX series had one as well and it worked fine.

“I have not failed. I've just found 10,000 ways that won't work.”