asterisk users - Aug 2017 - Odd audio issue with video conference

We're experimenting with using Asterisk (14.6.0) for video conferences.
This test has three endpoints, a Polycom Trio with its video accessory,
and two desktops running Linphone.  The video is all H.264.  We're using
Opus for audio on the Linphone Windows desktops and have tried both
G.722 and Siren14 on the Polycom.

When we have a two-party conference, everything works fine.  But when
we add a third party, it gets odd.  The two Linphone users can hear
each other just fine and the Polycom user can hear the Linphone users
fine, but when the Polycom user starts talking, all is OK for about
four seconds, then it gets replaced by a hiss for the rest of the period
of talking, then goes back to being OK and repeats.

This is peculiar.  It doesn't sound like an Asterisk bridge issue because
it only happens for one particular participant, but it's hard to see
how it can be a Polycom issue since it can't tell between two and three
participants.

Does anybody have any ideas here about what to try next to see if we can
diagnose this?