During the initial Sync, a Global (Org-wide default) Dial Plan is created in the Microsoft 365 Tenant.
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 this are Normalizaton 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:
The initial rule is suitable in most case scenarios, but customers may have reason to add a dial plan or custom normalization rules.
For example, where users are unable to dial certain numbers or if specific users need restrictions.
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
Rules can be added and amended but it must be remembered that a Sync will re-create any default rules that have been removed. If you are not au-fait with regex patterns it would be worth getting grips with how these work. Fortunately, the MS Teams Admin center allows for testing of rules before they are applied.
The following will allow you to create an international dialing rule. Rule creation mode must be set to advanced for the fields below to appear.
There is more information on creating Dialling Plans and Rules here: Microsoft Dialling Plans & Normalization Rules