Whilst you may be hearing the echo, it is not your headset that is the problem. You voice is going out to the other participants, getting coupled between one or more of their headsets and microphones, then returning to you.
It is their headsets that have to be sealed from the microphones to completely eliminate echo.
What I think Ian was saying is that, normally, the conference is set up so that the round trip time, from the conference bridge, to each participant, is the same. Assuming that the audio feedback delay is also small or consistent, the combined echoes can be treated as a single echo, by the phone’s echo canceller.
If, however, the round trip times change (e.g. because a latency buffer overflows or underflows) for one participant. The echoes generated at that participant will arrive at a significantly different time from those from the other ones, and the phone’s echo cancellation will have difficulty coping with it.
(I don’t know if the conference bridge has any specific tricks to try and maintain matched round trip times.)