Other party hears echoes when I use FaceTime for phone calls on Mac Mini

Other party often hears echoes when I use FaceTime for phone calls on Mac Mini.

It is an intermittent problem; some calls work fine, others have serious echos. That suggests my use of a separate mic and external speakers is not the problem, as some have suggested. Echoes occcur on both incoming and outgoing calls.

Thoughts?

Dick