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I am new to this community. I don't know if my question is On topic or not. I am dealing with a GSM modem. This is a kind of modem which works exactly like our phone. It is mainly used for call dialing, messaging and other applications similar to our phone. It works on AT commands. The modem has separate pins for mic and speaker indicated by MIC_N MIC_P for Mic and SPK_N SPK_P for speaker. I have connected below speaker and mic:

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Now the problem is whenever I am on a call, it echoes back my voice through speaker. There is not connection between speaker and mic because for them different pins are available. What could be the reason for echo.?

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That is called "sidetone". Sidetone is used to confirm to the phone user that their own voice is being picked up. Virtually all telephones use sidetone. A telephone would sound strange or even broken if you didn't hear the sidetone.

It sounds like you are trying to make a "speakerphone". Where the microphone can pick up the audio signal from the speaker. Speakerphones typically use special muting and anti-echo functions to tolerate this acoustic feedback loop. Your GSM module by itself was likely designed for common handset use may not be capable of implementing a speakerphone.

You might be able to adjust the sidetone of your GSM module to turn it completely off. That may be sufficient for the purposes of your experiment.

Ref: https://en.wikipedia.org/wiki/Sidetone

Ref: https://en.wikipedia.org/wiki/Speakerphone

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