If a Competella CC Lite user receives a call and attempts to transfer to an entered PSTN number (for example +4577337171) from his/her Teams client that transfer will fail. This applies to both hot (conference) and cold (blind) transfers.
However, if the CC Lite user switches internally to a Teams user who has simultaneous ringing or call forwarding enabled, the switch will work - however, this only applies if the switch is cold (blind) and not hot (conference).
The bug has been identified to be in Microsoft Teams API that Competella uses. The issue has been reported to and accepted by Microsoft, who considers it a bug in their API.
Microsoft is working on a fix.