Random Hold Music activating during middle of call and other side hearing the person

Ok in 5 years I never heard of such a thing but i got the strangest issue happening and my client says it happening several times and with different callers.

Basically my client is on the phone. all of a sudden during the call he right away hears hold music and can no longer hear the person on the other end of the call.

The person on the other end STILL HEARS my client. and my client meantime cannot get out of the hold music or anything so he has to hang up and call the person back.

Has anyone ever heard of such an issue? VERY strange