Just a quick question before proceeding to implementation.
Can a 3cx on-premise with Teams enable can be in a DMZ and not Windows domain-joined/Workgroups?
3CX needs to be able to access the internet to reach the Teams Servers. Also, the Teams Servers will need to be able to reach your 3CX Server.
If you can ensure this unobstructed access in both directions, 3CX Teams integration should be possible.