SIM900 speaker volume


I am using SIM900 module connected via serial to Arduino board. On positive speaker pin from SIM900 I connected a DTMF decoder and the problem is that the sound would be more powerful because the DTMF doesn’t receive every time. If I connect a preamplifier between SIM900 and DTMF everything is fine.
So, what’s the command to set the volume to max on speaker output?

Best regards