Thanks for the answer.
I didn't want to record audio in the SRAM. I plan to eventually record it on a SD card if the speed of the SPI bus is enough, but my problem now is that the modem doesn't send the audio at all.
I attached the only document I have about the modem's chipset... It includes all the information about the commands that the modem accepts.
ATCOMSET.DOC (562 KB)