Sorry but I completely fail to see the logic behind this.
What sort of modem?
In days gone by, a modem was usually a stand-alone box, it had two ports, the local data port which often used RS232, and a line port which uses some protocol such as BELL202.
In more recent times, the common perception of a modem is a card which plugs into a PC motherboard and has a line port which connects to the telephone network, using a variety of protocols, none of which are RS232.