Hi @Mora,
I'm afraid that the issue is an Office 365 one and not a 3CX one. I've googled and found various open threads, one bug reported to them was actually exactly what you wanted: Working Elsewhere showing as "Free".
As far as 3CX is concerned, you can find our mapping in the Management Console >> Settings >> Microsoft 365 >> Calendar Sync.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.