Clearly there is a difference in the PC <-> modem connection than you have in the MCU <-> modem connection if the PC works but the MCU does not. You need to carefully evaluate what is different at the modem. Use an oscilloscope to check signals at each pin of modem connector. Any differences need to be accounted for. It may be necessary to add port lines through the other rcvr/driver pair of your MAX232 to let the MCU control levels on the modem handshake lines. It may be necessary to re-config the modem to a subset of its current setup (ie disable handshakes) so it will work with the MCU. You may try getting the MCU talking direct to PC first. If you get this working then going to the modem from the MCU may be easier.

Have you checked the simple things that so often are what keeps RS232 ports from working.....ie...Are TxD and RxD going to/from the right places?....Are the baud rates correct?

When working on a problem like this you can assume nothing. Check every step....never make a leap of faith that just because one thing works that the next thing should work when you introduce a half dozen new variables.

Michael Karas