Recently a customer reported an issue where users attempting to join a conference and were getting a busy signal and seeing an error in the meeting saying they were already in the call. They could not join the audio portion of the meeting.
When looking at the Front End Server that was hosting the meeting you could see these events being logged:
Log Name: Lync Server
Source: LS SLA
Date: 4/7/2017 10:50:38 AM
Event ID: 61001
Task Category: (4040)
All the lines configured for the SLA group are busy
SLA Group: sip:Collaboration@contoso.com
Number of lines configured: 2
The clue here was the reference to SLA highlighted above. It turns out the account used to schedule the meeting was also configured to be a Shared Line Appearance. This shared line appearance was setup with 2 lines which is why no more than 1 person could join the meeting.
As a quick test we raised the number of lines available to the SLA by the following:
set-CsSlaConfiguration -Identity sip:email@example.com -MaxNumberOfCalls 25
After this, other users could join the conference.
However, I would not advise using an account that is setup as an SLA to host meetings for supportability reasons or other unexpected issues. If you do plan to use the SLA functionality in Skype for Business, you should dedicate an account to this and not use it for any other purpose.