From the Teams client to an Extension Number of a user that has the 3CX Client?
From the Teams client to an External Number?
From a 3CX Client to a Team Client?
What version of 3CX do you have installed?
Also have specifically gone through the Teams FAQ document we have created here? https://www.3cx.com/docs/microsoft-teams-integration-faqs/
It contains most of the common mistakes that admins do and usually most of the reasons are already included in this document.
In that case, the Teams integration is just fine, the issue is with your SIP Trunk and I am willing to bet it is because Teams transforms External Number into e164 format and your provider might not be accepting this.
Can you tell me which SIP Trunk provider you are using? It it one from our supported list?
When you dial an External Number from the 3CX Client that works, can you give an example of what format you dial with?
Also please post a screenshot of your Outbound Rules.