This article describes the use of audio recorder/playback IC's APR9600/APR33A3 for playing recorded sound during a call. The call is made using a SIM300/SIM900 GSM modem. The audio interface for SIM300 and SIM900 are same. So, The below description should work with SIM300 and SIM900 modem as well.
Preparing the audio clip : The audio to be played is pre-recorded in the APR9600/APR33A3 chip. Make sure you record the audio in a noiseless environment. The microphone interface which comes with the APR development kit is not so efficient at suppressing noise and amplifying the input. So, I suggest you record the audio in an audio player like a mp3 player/iPod or PC. Then record the audio into the APR memory by playing it at a higher volume.
Connecting audio to SIM300 : SIM300 comes with all the interfaces for making a call including the MIC input and the SPEAKER output. The audio output from the audio playback chip is connected to the MIC input of SIM300 modem through a current limiting resistor/potentiometer. Resistor is needed prevent the audio output from APR from saturating the MIC input of SIM300 and also helps in controlling the volume.
Connect APR9600/APR33A3 audio output to MIC+ on SIM300
Connect APR9600/APR33A3 GND to MIC- on SIM300
Making a call : The call is made from the SIM300 modem by issuing the "ATD" command.
Playing the audio : Audio is played from the APR chip by enabling the PLAY button.
If you wish to play audio after receiving a call, then look out for the RING command sent by the modem whenever an incoming call is detected. Then lift the call by issuing the "ATA" command.