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I would like to understand how to handle digital voice on gsm modules.

After reading 3gpp gsm mux 07.10 specification it is my understanding that gsm modules can be configured to transport voice frames on dedicated channels (DLC) sending a SNC frame with bit3 set on.

I considered to use simcom sim900 module but official documentation indicates that SNC frames are not supported.

Are DLCs channels the right approach to send&receive voice frames? is there another way to do it (PCM, PWM, I2S interfaces)?

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My other brand GSM module has analogue in and out pin. Its take MIC IN and handle the coding and transmission inside the module.

Data sheet of your module has similar wording. may be, it work in same manner.

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GSM uses human speech coder. It 'cannot' be used to transmit free format AF frequency signals, like feeding an analogue modem to the MIC in, as in phone line of plain old telephone system. Human vocal muscle has speed of about 10 to 20 changes per second. The coder will only encode speech or speech-like AF signal at repeating frames in which signal remain constant in each frame (constant for about 1/10 to 1/20 second) –  EE developer Dec 23 '12 at 14:21
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