Hi sharewq!
Does this issue persist all the time?
How external users join the meeting?
Based on my knowledge, external users have two ways to join the meeting. First is the external user using their Skype for Business client to join the meeting. These type of users come through your Edge configuration. The second way is to join the meeting
using https://meet.companyx.com/UserName/meeting ID?SL=1. these users join meeting via RP 443 and then reverse proxy to 4443 to your FE Server.
Considering this issue related to external users, we recommend you check your external DNS configuration on Edge Server. You can refer to the following pictures to see more details:

If you cannot join meeting in the second way, also please make sure that you do not use ?SL= at the end of the URL as an organizer.
For more details about conference process in Skype for Business, you can refer to the following picture:

Best Regards,
Jimmy Yang
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