It can happen on any service and it means the echo cancellation on their end got confused -- it's an adaptive algorithm but occasionally adapts wrong.

Best way to fix it is to mute yourself for ~5 seconds and then unmute, which should reset their echo cancellation algorithm.

If that doesn't fix it, then have both of you pause and not talk for ~5 seconds. This should absolutely reset it, as the echo cancellation algorithm has now definitively learned what silence is supposed to sound like.