You have a user who, despite being able to make/receive Teams to Teams calls and receive PSTN calls, cannot make any outbound PSTN calls.
There are several potential causes of this issue, and these steps will help you to diagnose the issue so that you can take corrective action. Start by checking call status indicator on the Users page. If it is Red, the issue is most likely between our system and your PBX or Trunk. If it is not, then the issue is more likely between our system and Microsoft.
1) The Call Status Indicator is Red
On the Users page, there are up and down arrows against each user that highlight any users with failed calls. Hover over the arrows for more information. If the up arrow is red this means we tried to relay the call on and failed, and this should give you an indication of why. You should check that your PBX is not rejecting calls for any reason, such as, for example, the password does not match, or it is not configured to handle E164 formatted numbers.
1a) Timeout error.
This error means the service had no response to the outgoing SIP INVITE. This could be firewall related. See here
for further information on white-listing IP Addresses. if required by your PBX/Trunk.
1b) Connection refused
Our attempt to send a call to the PBX/Trunk was refused. Check your PBX transport matches the expected settings on your firewall.
1d) See Also
2) You See an Unmatched Calls Warning
If there have been calls from Teams that did not match any of the users configured in the portal then this will be highlighted in red at the top of the Users page. This means the phone number sent by Teams didn't match any of the users you have configured in the portal.
Check that you have enough licences, and that the phone number configured in Teams matches the phone number configured in Office365.
If you have enabled Teams Sync , then click the Sync button to run another sync to make sure the numbers match.
3) Sync Errors
If you have enabled Teams Sync functionality you should check to see if there are any Sync Errors. If there were errors then the Sync button will show red. Click the drop down arrow on the Sync button of the Users tab:
You can expand the log using the small black arrow to the left of each entry.
If you do not see the Sync button, you have opted to configure Office365 manually.
You can find further configuration information either in the contextual help links throughout the portal or via this support portal.
If you have exhausted all of the above options without success, then please raise a support case and provide logs and/or screenshots.