MOH is heard in the second conversation

I have most of the things working fine. One annoyance. When I put somebody on-hold and switch to another caller, I can hear music-on-hold about one-third of the loudness of the conversation. I have never seen it discussed, and have no idea where to start troubleshooting!
As I said elesewhere, I am running PIAF in RentPBX. Not sure if it is PBX or the phone problem. PIAF crew has more pressing problem to deal with at the moment; so, hopefully somebody can give me some guidance here…