@amy.blais thanks for keeping an eye on this problem .
As for me, looks like WebRTC could not handle a connection because of some external reasons. This probably does not depend on the Mattermost platform. May be it is a NAT or STUN issue. At the same time even a browsers could get some restrictions to provide video/voice stream for these 6 months after last WebRTC release.
So I am very waiting for this issue to be solved.