If a user finds they cannot make or receive any calls on their Teams device the first thing to check is whether they are still registering correctly.
See this article for more information: User Registration Check
If this is not the issue then you can check the status of recent calls from the User Tab:
We can see, straight away, that there has been a recent issue with a call to or from A. User. By hovering over the red call arrow we are presented with a snap-shot indication of the problem and can click the View Recent Call Log link to open a dialogue box with more detail:
From here we can launch a more in depth analysis of the issue by clicking on the icon highlighted in red above.
This launches in a new window and provides much greater detail which can help with diagnostics of the problem or shared with your supplier as needs be.
Bear in mind that there are several elements in play between the connector, Microsoft and your PBX so this data can be very useful.
You can read more on outbound call issues here.
You can read more about inbound call issues here
It is always worth double checking that Microsoft, your PBX and your Teams settings all match and the correct licences are in place.