As a first step I would recommend to study the AT commnads of the specific phone.

Maybe it needs something at the beginning to enter AT command mode.

Then, please, DO connect the phone to a PC directly and test it with a Terminal program.

After you feel confident with the phone then go for the micro controller connection.

At this stage you do not know what the phone sends as a response. Maybe instead of 'OK' sends a 0 (zero).

Ioannis