All methods of altering the audio sent and recieved by the GSM module.
This basically means putting the CPU into the loop, which will of course use power.
There are various simple options - listen to both sides of the call, and the ambient noise, and play with the mixer levels to keep things right.
You probably want to add a ear sensitivity curve, so you get it more or less right, for voices of various pitch.
Then there are more complex applications.
Pitch bending - making you or the incoming caller sound lower or higher pitched, to aid in clarity, or for people whose voices grate on your ear at their normal frequency. (per-caller of course). This may be especially usefull for older people, who often have age related high frequency problems.
Vocoder - 'joke' voices - though sharply limited by what GSM will let you send sensibly.
Subtle voice changes - suppressing the sibilants slightly, to gain a sound that's not quite as 'telephony'.
Some of these require 2 analog-digital and digital-analog converters to be active at the same time, and for the mixer to be able to seperate the channels in the appropriate ways. This may not be possible on V1 hardware.