Applies To

Call2Teams - Customers


This article covers a number of extension numbering scenarios including the use of Common DDIs where only extension numbers are available to users who do not have their own DDI number.

We will also look at assigning numbers generally and using extension numbers in Call2Teams where the string length exceeds the default 6 digits.


Assigning Numbers - General Note

You can use the extension numbers in the Teams User profiles in O365. Any number assigned to a user must be unique within the customer

 You can manually input the extension number into the Call2Teams user settings (ignore the country setting at this stage):

Trunk Users

A trunk user will need to have a number assigned. You cannot add a Trunk User without a number. If your trunk does not have a sufficient number of DDIs in the range(s) specified, you can add a dummy range of (4-digit or greater) extension numbers. These can then be assigned to users in place of DDI numbers.

Note that you may wish to consider how to distribute calls from DDI numbers to these users. This can be achieved by creating users as target groups in Teams where several users are group members.

Can I Use A Common Caller ID DDI For Several Teams Users?

This can be achieved via the Microsoft365 admin portal. See this article: Setting Caller ID

Withholding Your Own Caller ID

In the Teams client you can withhold your own Caller ID:

You then have the following options

A PBX user may be able to use features in the PBX to present an alternative Caller ID (such as a group target number)

A Trunk user can present an alternative Caller ID via the From Header setting in the Trunk Settings page in Call2Teams:

Extension Number Length

By default, Call2Teams treats dial strings whose length is 6 digits or greater, as an external call. Here is how you can allow for internally routed extension numbers of 6 or more digits in length:

The summary is to duplicate the Call2Teams normalization rule within the Global dial paln and then modify the duplicate to match longer numbers.

  • Start by logging in to your Office 365 Admin Portal and Select Teams Admin Center, then on the Left Navigation go to Voice 








  • Dial Plan > Click Global. You should now see the normalization rules. Call2Teams creates a rule that allows feature codes (dialed numbers that start with an * or #) and short numbers to be passed through as-is to the far end. Short numbers are considered extension numbers and so we match them with a rule to avoid Teams converting these to E164.
  • Click Add by the normalization rules table. Enter a suitable name and description, perhaps mentioning that the purpose is to match larger extension numbers.

  • Select 'Advanced' to allow use of a regex.
    • The 'If' condition by default is: ^([\*\#][\*\#\d]+|[1-9]\d{0,5})$
    • If you are unfamiliar with regular expressions the important number is the 5 near the end. Increase this to 6 if you want to match 7 digit extensions (note we assume extensions start with an explicit 1-9, which is why this is one less than the total number length). For example: ^([\*\#][\*\#\d]+|[1-9]\d{0,6})$
    • The 'to' condition should be $1
    • The existing normalization rule should be left untouched as this will be checked by Call2Teams sync and amended if different from the expected default.
    • Ensure your new rule is above it on the global dial plan, .e.g below you can see new Normalization rule is created as “My Company rule” and the dial plan is set for the Teams users to dial numbers with a maximum of 7 digits



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