Thanks to another poster mentioning something it dawned on me, the pay telephone moved around in that diner didn't it? It is first on the wall by the counter and where the doors swing in/out. Then later, it is seen by the far end wall by the last booth.

Also, later on in the seasons, the back room is shown several times and there is a phone, am I correct? So, why did the ladies use the pay phone rather than the regular phone in the back room?

That's a good point.

Remember in the episode where Nedra Volz was the guest star and she was breaking into the diner and stealing flour and other things and using the phone?