I have one of the pay phones from payphone.com and it's just hooked to an analog port on the PBX. It's not like the traditional pay phones from the carriers that generate tones and have special resistance - it does all of its own coin counting, etc for authorization. It allows you to dial the number but will not complete the dialing sequence until the money is inserted, and then (generally) will not drop the money in the bank until someone answers and speaks.
I have had no issues.