Sound level is variable from call to call


I’m calling from a Linux PC to Android Phones and the sound level is not consistent, with or without video. For some calls, the sound level is reduced on both sides, and even if I max the sound level on the Linux side (the Android is always maximized and using the speaker does not change the level), it is faster to stop the call and wait a few seconds before starting a new call with hope of better sound.

What could be the cause of such behaviour? What can I do for consistent sound quality?