Below, we will look at the most common reasons and attempt to offer some guidance on analyzing and resolving them. 


First Checks

If a user finds they cannot make or receive any calls on their Teams device the first thing to check is if this is a Microsoft issue.

Real time updates of Microsoft 365 service issues are available at: https://twitter.com/MSFT365Status

If there in no relevant alert from Microsoft then continue with the checks below.

Has the affected user successfully made/received calls previously?

Are they successfully registering to their PBX/Trunk?

Are their Microsoft Teams User credentials still valid?

Has anything on the user's Microsoft profile been amended, such as a new calling plan or a licence change?

Has the user's licence expired on our portal or has it been removed?

Can you see the errant call against the user in the portal?


In almost all cases, it is worth taking a look at the call log for the user affected. Using the example above, 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.

At the top of the Call Data page, a failed call will present a Failure Advice similar to the following:

Below this are the call details:

And below that is the Debug Output.

Bear in mind that there are several elements in play between Call2Teams, Microsoft and your PBX so this data can be very useful.

Identifier is not Linked To A Service User

Basically the identifier in the user profile (usually the number) cannot be matched to a user profile in MS 365
You may find that the MS365 entry for the user does not match the number@domain format expected.
This may be something on the MS side or an issue  with your Sync module.

You may need to wait on Microsoft if the user has recently been added. After an extended period (in some extreme cases, days) you may be advised to remove the users, or clear the user settings in our portal, and then re-add/re-sync.

No Incoming Calls

  • 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.
    • You can adjust the E164 formatting for calls through the portal PBX/Trunk Service page.
  • Not Found (404 Errors)

    • This usually means that the user has not fully completed registration on the Microsoft side as is often the case with new users. Please allow up to 48 hours to register depending on demand/load on Microsoft.
    • Occasionally, we see cases where a user's number has been amended in Microsoft 365 but has not fully propagated through to Call2Teams (or vice-versa). A new Sync will often resolve this, but on rare occasions, numbers may need to be removed either on the MS side or the Call2Teams side, or both, and then re-entered, followed by a subsequent sync. 
    • There is more on 404 errors here: Unallocated Numbers (404 errors)
    • In some cases, this issue can be attributed to tenants who have both Teams and Skype for Business running at the same time. (Islands Mode). Here is a Microsoft article that explains this in greater detail: Teams & SfB Coexist & Interop 

Unable To Dial Out

  • Timeout Error

    • This error means the carrieer 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.
  • 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.
  • Unmatched Calls

    • 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.
  • Call not allowed due to your organization's settings.

    • If when you try to dial out any number you are receiving audio message from Microsoft Teams saying "Call not allowed due to your organization's settings" then review the following as a possible cause:
      local based routing is enabled once PreventTollByPass is enabled on user.
      assign a TeamsCallingPolicy to users to AllowCallingPreventTollByPass
      Since the Prevent Toll By Pass may be blocking the call then change the policy to AllowCalling
    • This guidance was taken from this Microsoft community post:

No Response To The First INVITE

For each new call sent into the carrier the SIP INVITE packet is mapped to a corresponding user. If you are getting no response to your INVITE this means we couldn't map to a single user, or the INVITE is ambiguous and maps to multiple users.

We primarily find the correct "User" based on:

  • Username (in request URI)
  • Source IP (which needs to match the Registration source IP or allowed IPs if configured separately)
  • and From Header domain (when the above two match with multiple users)



Below is the example where the carrier did not send back a Response to the INVITE. 


This is because, we initially matched up to the ”Username & Source IP” and as it did match with multiple users we went back to match the “From” Header Domain however, the Realm of the customer PBX was sent only in the “To” Header Domain and not in the “From” Header Domain which meant we couldn’t recognize the User on our end.


Calls Drop After Connection

Calls may drop for a number of reasons. During a call there are several exchanges between the PBX/Trunk, our SBCs, Microsoft and the external parties. Each component part of the process needs to keep alive and will often check on progress. It is not, therefore, uncommon that secondary and subsequent INVITEs are sent and responses expected. When checking the call logs or progress through a PCAP, it is worth establishing whether all CODECS are being included for each invite and that each message receives a valid response. We often note that PBX's or Trunks do not respond to additional invites or 180/188 Ringing messages. We also note that on occasion, responses are sent on different port numbers. Please check before reporting.

Missed Call Notifications

Teams registers a Missed Call when a call is answered by another device on the system.


This behaviour is usually due to a setting on the PBX.

Where several devices are assigned to one user profile the PBX will attempt to distribute the calls to all devices depending on an assigned policy linked to the user profile.

This is rather like being a member of a Hunt or Distribution group with several other end users.

The call, having been answered by one endpoint will rise a flag in the PBX which then sends a new message to all other endpoints to say the call has been answered.  this will generate a Missed Call alert


In the PBX settings there should be a setting you can set which basically turns off the Missed Call alert. The setting may say something like Missed Call Alert: Ignore if answered elsewhere. As a result, the PBX will send the following message to the device: REASON: SIP; cause=200;text”Call completed elsewhere

This advises the device NOT to flag this as a missed call.

Further Reading

Please click the link below.

Missed Call Notification - Teams

Error Codes

There are numerous error codes that are often presented as part of SIP messages. Please review our KB article on SIP Error Codes. These are very useful in understanding what actions need to be taken to ensure the issues being experienced are resolved.