Bubble nois when calling

For a very long time my phone has been giving problems with sound, done a fixit now it is good (not loud but good enough), only when calling with regular connection with 1 person (my son), I get an underwater bubble sound. Calling via Whatsapp does not give this sound. Also I can fix it by restarting mn phone completely. It is my son, so I call regularly, always having the same problem.

He thinks it is due to the FP, because has the problem only with me… He can call with others just fine. Me too.

Does anyone recognize this crazy problem: bubble sound / complete unintelligibility when calling - with the normal connection - between only two phones and fixable with a restart of my phone?