Overview

A Global (Org-wide default) Dial Plan is created in the Microsoft 365 Tenant during the initial Sync. 

Dial Plans and Rules can be found in the MS Teams Admin Center under the Dial Plans menu:

The default plan for the service should appear like this:

Beneath these are normalization rules - there will be at least one - that are processed in the order presented. 

These contain call routing options based on a Regular Expression (Regex) pattern:

Usually, the initial rule is suitable, but customers may have reason to add a dial plan or custom normalization rules. 

For example, users cannot dial certain numbers or if specific users need restrictions.

Synchronization

The dialling rules and policies are applied at the initial Sync and are checked (and reapplied if missing) with each subsequent Sync. 

An example of this check can be seen below:

<00:07> PS> Get-CsTenantDialPlan -Identity "Global" -ErrorAction "SilentlyContinue"
<00:08> Confirmed existing Tenant Dial Plan

Adding and Amending Rules

Rules can be added and amended, but it must be remembered that a Sync will re-create any removed default rules. If you are unfamiliar with regex patterns, it would be worth getting to grips with how these work. Fortunately, the MS Teams Admin center allows for testing rules before they are applied.
 

There is more information on creating Dialling Plans and Rules here: Microsoft Dialling Plans & Normalization Rules.