When Rebecca calls her mother on the payphone, the phone is only held to the pole with zip ties and there are no cables connecting it back to the telephone system This shows that the phone was simply a dummy prop.
After Johnny and Rebecca leave The Blue Swallow Motel to get out of town, Rebecca takes the car to go call her mother. The payphone that she uses (at the Motel Safari) is less than 1 block away from The Blue Swallow Motel. The motel sign is clearly visible between the pillars in the background on the left side.