You didn't specifically say that the RS-232 port is available but if it is I would suggest using the 9300-RADKIT. You could use other serial modems but the advantage of using these is that they always work the first time as opposed to fighting to get the configuration properly setup.
Once you have these properly communication over the telephone lines communications should be the same as if they ware talking from serial to serial. As for the details of doing this someone else will have to help as I have limited experience with the PLC5