good point amit... the first thing is....we didn't use actual modem inside proteus, compim was just a connector to connect the microcontroller to external devices. and yes in actual hardware the connections are different and are according to more common conventions that you would find in a number of books. i.e to connect Tx(transmit pin) of microcontroller to Rx(receiving pin) of modem and vice-versa. well it is very easy to remember this convention as the signals sent by microcontroller would be received by modem and those by modem would be received by microcontroller. now in proteus, I think that compim connector serves the purpose of an RS232 port for microcontroller side(other members please explain this if am wrong here) and thus one needs to connect same pins of microcontroller to its corresponding pins. although if one connects the actual hardware...he would need to connect in more common way, i.e Tx(microntroller) to Rx(modem) and vice-versa.