there are a lot of devices out there that could do voice over I.P without a computer. They look like a modem and go connected betwen the phone and the wall jack.You may have one at each end and the call is almost free( only the cost of the internet access on both ends) or you could have only one on the originating end of the call and paid a company that provides internet telephony (ITSP). That normally runs few cents per minute.
Those devices are programed using the phone and a “cheat sheet”