Rod Speed explained :
Never seen that and I have been on hold quite a bit
lately due to changing various service providers, in
fact due to changing all but the local council with
some glitches with every single one.

When ringing such organisations, I normally put it on loudspeaker. The
reason I noticed it, was that the music would constantly break up. I
traced it to the phones mike picking up noise in the room from the TV
and me moving about. I then progressed to experimenting, by tapping the
mic, whereupon it would mute and covering the mic where the music would
play without a break. The voice announcements were never muted.

It was annoying in as much as I thought when it went quiet, I was about
to get put through. Next time I need to ring, I will resort to a bit of
bluetack over the mic..