I need to adjust the gain for voice messages that I play and record. The messages are barely audible to a caller over the phone, how can I set these levels?

There are two commands to look at:

The AT#VGT command sets the transmit level of output to the phone receiver. You can increase this parameter for a higher output to the phone receiver . The default is 129, the range is from 128 to 255.

There is also an AT#VGR command which may not be listed in the manual but has the same adjustment range, this sets the recording level.