Who called Alice that second time?

Alice is talking to her mother. Then later the phone rings again and she assumes it is her mother but it isn't. There is nothing on the other end of the phone and I think audiences are supposed to expect it to be the killer. Except for one thing, this is 1981. No one has a cell phone. You called through a pay phone or through your own home. Are we to expect it to be Jason on the other end of the line? I just can't see how that could be. Jason was outside of her apartment at this time.