Modem Connection to Siemens PLC

This would be a PPI connection which is possible via a modem connection. It is difficult at times getting any Siemens protocol to work via modem, but a leased line should help. These protocols have time dependent packet transfers that can cause more errors then would normally be expected. Siemens can also support a Modbus protocol through their ladder logic. If the PPI protocol is not stable enough for your situation, you should look into the Modbus protocol.