I’ve heard about this problem many times - seems to affect some users, no matter what phone they use (not FP or Android specific). Sometimes you get an echo yourself, sometimes it’s the person you talk to who hears an echo.
Various causes have been suggested: Casing picking up and transferring sound from speaker to mic, software issue, hardware issue, network issue.
Eliminating the phone case is of course the easiest. Do you use any kind of case? Do you experience a difference after removing it?
In System Settings > Audio profiles there’s an option called BesAudEnh (Audio enhancer), disabled by default. Have you enabled it? Then disable and see what happens. If disabled, enable and see if it makes a difference (maybe less likely).