There are several reasons why an inbound call may fail to reach your Teams device.
In the first instance, please read this article: Inbound/Outbound Call Issues
Shown below are a few examples for why you may not be receiving calls. Often, looking into the call logs as described in the linked article above, will shed some insight into these.
1) PBX Registration Timeouts
If your PBX registration times out, your Teams device will not be available to receive calls. This may be noted as an intermittent ability to receive calls.
Hover over the Registration indicator in the Users section of the portal and check the 'Next Sync' advisory. Consult your PBX administrator if this issue persists.
2) 404 Not Found Errors
Inbound call to Teams may encounter a '404 not found' error. This is most common when an account is new. The reason is that the user data we share with Microsoft can take some time to propagate through their system. The best solution here is just to wait. This can take an hour or so in some cases but is often much quicker.
Occasionally the Microsoft platform is out of sync or an admin has changed the user's number in the Office 365 portal, this will cause the Microsoft Teams number associated with the user to be different to that set in the service portal and produce the mismatch. To resolve this, set a random/different number for the user in the service portal and sync, then set the correct number and sync again. This will ensure the correct number is stored against the user in Microsoft Teams. It may be necessary to allow Microsoft a little time to catch up with the changes made, for half an hour or so.
A less common cause of 404 errors can be attributed to users having both SfB and Teams running in Islands mode. Here is a Microsoft article that explains this in greater detail: Teams & SfB Coexist & Interop
However, if you believe there to be a genuine error, please raise a service ticket with your partner. Also please check the user has been synced from the portal (Sync Now) as sometimes users are added/deleted and the re-sync is overlooked.
404 errors on outbound calls from Teams
If you are calling from Teams and the PBX or Trunk is returning a 404 error, then the most likely cause is a number formatting error or and E164 error. Please refer to this article for more help:
3) 403 Permission Denied: No match for Service User Part
This error usually means the number offered by the PBX or Trunk for a user is incorrect. It is most likley to be caused by E164 number mapping issues. refer to this article for help: https://connectorsupport.freshdesk.com/support/solutions/articles/80000127095-e164-formatted-number-translations-403-errors-
4) 504 Couldn't Classify Packet or No Response
This is likely to be a mismatch between the IP address used when service registered with the PBX, and the PBX IP address being used for the call delivery.