Hi @IBN ,
When we try convert the license to MTR we having issue with unable to fetch calendar on our conference system.
By this, do you mean you encountered the "Cannot Fetch Calendar" error?
Are you able to see the meeting room calendar entries if signing into the Teams desktop client on your own computer using the meeting room credentials?
If the license has just been converted, please try to wait for a few more hours and see if the issue persists. I've seen discussion under other similar threads that it may take up to 48 or 96 hours for the new assigned license to work properly. See this link.
In addition, this thread mentions that the error was finally resolved by enabling Modern Authentication in the device's settings. You can also have a check to see if there's a similar settings on your end.
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