sang,
My experiences lead me to think that remote monitoring of a PLC is great, but that remote control can be dangerous. You did say "controlling" a PLC. I recommend actual remote control only where you also have very good remote monitoring of the process, so that you can determine remotely what is happening, or where you have a local operator or technician that can tell you (via phone) what happens when you make a remote change. Otherwise you are flying blind. That said, there are many systems. Rockwell RSVIEW32 has the remote capabililty for an operating system, called the "Active Display System". Also the RSLogix PLC program has an optional dial-in modem that can be connected directly to the SLC 500 PLC's.