"I ve designed a user interface in Visual Basic by using MSComm control from where I can dial telephone. Is it possible to control the 89C51RD2 projects?"

Of course it is!

You now know how to talk to the PC's serial port from VB - so you can talk to anything that's connected to the COM: port - whether it's a modem, a terminal, a datalogger, an 89C51RD2, etc, etc, etc, ...

Similarly, once you know how to use the 8051's serial port you can use that to talk to anything with a serial port!

NB: hardware-wise, you will need a MAX232 or similar transeiver between the 8051's serial port pins and the COM: port on a PC (or other RS232 device).