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 errors when calling into Teams (RNL)

Please review the separate article 404 errors when calling into Teams (RNL) 

3) 404 errors on outbound calls from Teams

When us user is fully configured within Teams, has the dial-pad in their client but cannot make calls outbound (to the PSTN or PBX), then review the call log.

An 404 error may indicate a configuration issue on the PBX or Trunk:

Review the Call log for an error of the following format (exact wording of the error may vary):

2021-05-07 20:17:35 (UTC) SIP processing error Invalid argument 404 
2021-05-07 20:17:35 (UTC), new state => Closing Error, accepted => Y 404 Not Found

The 404 Not Found error is returned from the SIP Trunk or PBX and the logs available on that system will give more insight as to the reason. Consult the administration or support team for that system to get the logs reviewed.

The most likely cause is a number formatting error or and E164 error. Please refer to this article for more help:

Configuring E164 and non-E164 Number Formats

Occasionally more information is available and returned from the PBX or Trunk in the call log, for example:

IP/2.0 404 Not Found
Via: SIP/2.0/UDP;branch=z9hG007f7a7621cda6bff5e32ff44ff24af5ca7c
Contact: <sip:;transport=UDP>
To: <sip:+12245559704@contoso.com;user=phone>;tag=0_1742321954-40322320795
From: <sip:bob1@contoso.com>;tag=819d0679bec87fb88b721efb
Call-ID: 999b0c54b626265b72ed55b2341bd677
Date: Thu, 06 May 2021 14:48:36 GMT
User-Agent: Mitel Border GW/
Warning: 399 "00H Number Unobtainable"
Reason: SIP;text="Forwarded response from LAN-side hop";cause=404
Content-Length: 0

Here we can see the PBX is helpfully provided more meaningful feedback that can be referenced in the PBX documentation to identify the solution. Any such information will vary between platforms, the example above is for illustration only.

4) 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 likely to be caused by E164 number mapping issues.  refer to this article for help: https://connectorsupport.freshdesk.com/a/solutions/articles/80000127095

5) 504 Couldn't Classify Packet or No Response

This is likely to be a mismatch between the IP address used when the carrier registered with the PBX, and the PBX IP address being used for the call delivery.