If you miss the MAX232 then send in inverted mode. The MAX does this inversion.

Another thing you may try is to use Numeric mode instead Alphanumeric. Then the phone answers 0 (zero) for OK, 4 for error etc.

Did you double check that you have not connected Rx, Tx the wrong direction? Try to cross connect them. Don't worry, RS232 is able to withstand this if it wrong.

Ioannis