Android - Way to prevent the noise when 2 phones together

This phenomenon is called audio feedback which usually occurs when the sound from the speakers reaches back into the microphone, and then gets amplified because of the speaker-mic loop.

Common and general solutions are to cut off the loop, or at least reduce the loop gain by:

1. Wearing earphone or headphone (isolate the speaker and the mic, prevent the feedback loop completely)
2. Mute the mic when not needed to speak (prevent the chance of feedback loop)
3. Reduce the speaker volume (reduce the volume gain)
4. Take some distance between the speakers (reduce the volume gain)

Before feedback cancellation techniques became common, such an effect would happen even with one pair of phones, e.g. phone A calling phone C. If both phones are on loudspeaker, then the signal emitted by the phone A would go into A's microphone and get transmitted to the phone C, where the same feedback between the loudspeaker and the microphone would re-transmit it back to A, etc:

SPK_A -> MIC_A -> SPK_C -> MIC_C

This problem is prevented by subtracting the loudspeaker output signal for the microphone input. However, for such a software subtraction to work, phone A needs to know what to subtract: if A is the only phone in the room, it can subtract its own loudspeaker output, and feedback cancellation works as intended.

With a second pair of phones, feedback cancellation fails because the signal emitted by the phone A is picked up by the phone B instead. As the phone B doesn't know what signal A was emitting, it cannot subtract it from its microphone input, and sends it to phone D:

SPK_A -> MIC_B -> SPK_D -> MIC_C

There is a second (symmetrical) feedback loop that is possible:

SPK_B -> MIC_A -> SPK_C -> MIC_D

As you can see, the loops are closed, and there is no opportunity for the feedback cancellation software to break those.

In order to prevent the noise, you'll have to break the cross-phone loops physically, e.g. by muting both loudspeakers (switching to earphone) in one of the rooms (e.g. A and B), or in one of the calls (e.g. A and C).

Indeed, muting the microphones, being in separate rooms or having a single call at a time would also help, but I suppose those are not real options.