Solution:

Actually, you need to get the dial up modem with auto-answering (AT command is something like: ATS0 &W&Y. In fact, most of the modem nowadays have this feature). If you have this dial up modem, all handshake is not needed - because after pre-programmed X - number of rings, the modem will automatically pick up and connect the PSTN to the RS232....therefore creating a "through" connectivity from PC---- to -----ADAM4520, via modems and PSTN.