The phenomenon you're referring to is known as "interference" or "audio feedback." When two phones get close to each other during a call, the speakers and microphones of the devices can interact with each other, resulting in strange noises or feedback loops.
This interference occurs due to the way sound is captured and reproduced by the phones' audio systems. When you speak into a phone's microphone, your voice is converted into an electrical signal and transmitted to the other phone. Simultaneously, the other phone's speaker produces sound based on the received signal.
When two phones are in close proximity, the speaker of one phone can transmit sound waves directly into the microphone of the other phone. This creates a feedback loop where the sound from one phone's speaker is picked up by the other phone's microphone and amplified again, causing the loop to continue and create the strange sounds.
To prevent this interference, modern phones employ various technologies such as noise cancellation and echo suppression. These techniques help reduce feedback and improve call quality, but they may not completely eliminate the issue, especially when the phones are extremely close to each other.
It's worth noting that these interference sounds are more likely to occur when both phones are actively in a call or when the volume is set at a higher level. Additionally, certain environmental factors or phone configurations can also influence the occurrence of interference.