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.

The article also covers assigning numbers in general and using extension numbers where the string length exceeds the default 6 digits.


Assigning Numbers - General Note

Customers can use the extension numbers in the Teams User profiles in Microsoft 365.

Any number assigned to a user must be unique within the customer.

Customers can manually input the extension number into the user settings: 

Ignore the country setting at this stage.

Trunk Users

A trunk user will need to have a number assigned. 

It is not possible to add a Trunk User without a number. 

If the customer trunk does not have a sufficient number of DDIs in the range(s) specified, users 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 customers 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.

Is it possible to use a Common Caller ID DDI for several Teams Users?

This can be achieved via the Microsoft 365 admin portal. See this article: Setting Caller ID

Withholding Own Caller ID

In the Teams client users can withhold their own Caller ID:

Customers 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 of 6 digit length or greater as an external call. 

The following steps show how to allow for internally routed extension numbers of 6 or more digits in length:

The aim is to duplicate the normalization rule our Sync process applies within the Global dial plan, and then modify the duplicate to match longer numbers.

1. Log in to the customer's Microsoft 365 Admin Portal and Select Teams Admin Center.

2. On the navigation menu, go to Voice.






3. Under Dial Plan, click Global. This will show the normalization rules. 

Our Sync service creates a rule that allows feature codes (dialed numbers that start with an * or #) and short numbers to be passed through unmodified to the destination. 

Short numbers are considered extension numbers and so it is possible to match them with a rule, to avoid Teams converting them to E.164 format.

4. Click Add next to the normalization rules table. 

5. Enter a suitable name and description. It is advisable to mention in the description section that the purpose of the rule is to match larger extension numbers.

6. Select 'Advanced' to allow use of a regular expression (also known as RegEx).

  • The 'If' condition by default is: ^([\*\#][\*\#\d]+|[1-9]\d{0,5})$

For users unfamiliar with regular expressions the important number that dictates the string length is the 5 near the end. 

Users can change this to 6 if they want to match 7 digit extensions.

For example: ^([\*\#][\*\#\d]+|[1-9]\d{0,6})$

Note: We assume all extension numbers start with an explicit 1-9, which is why this is one less than the total number length. 

  • The 'to' condition should be $1
  • The existing normalization rule should be left unchanged as this will be checked by our sync process, and amended if different from the expected default.
  • Ensure that the new rule is above the existing on the global dial plan, as shown in the example below:

The 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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