The laptop shows your camera and a phone beside it displays the shared slides. Then someone hears every sentence twice. Two devices in the same room can create a return path: one plays meeting sound and the other's microphone picks it up.
Muting is not always enough
Zoom lists multiple devices with active audio among the causes of echo. Muting the phone's microphone still leaves its speaker playing. The laptop microphone can pick up that sound and send it back to the meeting.
Keep one device connected to meeting audio in the room. On the second, disconnect audio rather than merely muting its microphone. Zoom's mobile guidance places Disconnect Audio under More in the meeting controls; labels and placement can vary with the app interface. Confirm that the extra device has stopped playing the meeting.
Make one change, then listen
Ask someone outside the room to listen to a short phrase after the change. If the repeat disappears, keep the second device disconnected from audio while using its screen, where the app permits. Do not turn up the speakers to overpower the echo.
If the problem remains, inspect any room speaker, television or soundbar still playing the call. A host can also help isolate the source by muting participants one at a time. Hearing your own voice return does not establish that your microphone is faulty: the return path may be elsewhere.
Work through the audio connections before changing network settings. A speed test cannot tell you which nearby speaker is feeding a microphone.