I have been able to write a program which can send String objects over the serial port using java communication. A server side code listens to the serial port and a client side code send data over this port. (I dial to a computer from another computer using one modem at each end)

What I require is a bit tricky though.
1. One computer dials another computer and the call is received.

2. The second computer on receiveing a call sends a file over to the calling computer using the serial port.