you normaly get a 3.5 mm mono jack in with the screws.
I Have used normal stereo leads but they can be hit and miss pulling it in and out to make contact. try mono leads as i think the socket in the system is mono
I have come across on a kxtda 100 that if the audio level is to high going in to system it can stop it coming through on hold.

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