What is RNL?
RNL or Reverse Number Lookup is a way by which Microsoft identifies the user based on the dialed string. Let's assume Alice dials +421177228813, when this call reaches Microsoft phone system, lookup for this number is made against a user on Teams. If it matches, call successfully reaches the user, else Microsoft sends 404 (Not Found) error to your PBX or SBC.
In a more complicated scenario, a telephone number of a Teams user might also hold an extension. For ex, +421177228813;ext=8813.
I will divide my further explanation into 2 use cases:
1) DID based Extension
Lets take the same example as above, the telephone number of user is +421177228813;ext=8813. Until now, when a call arrives to an SBC with called number as +421177228813, SBC sends invite towards Teams with destination number as, +421177228813. Microsoft phone system, starts lookup and finds the number +421177228813;ext=8813 assigned to user 'A'. However, the call is accepted and Teams finds user with the main number and ignores extension, and hence the call to user 'A' establishes.
2) Non-DID Based Extension
Let's Assume, +421177228813;ext=8813 is a number assigned to Auto Attendant in Teams for site 'XYZ'. The users at site holds the number in format, +421177228813;ext=8814, +421177228813;ext=8815.......................+421177228813;ext=8827.
The purpose of having such phone numbers is for the convenience of internal dialing. Inside the Dialplan for site 'XYZ', the normalization rule, assists the site users to dial 4 digit extension. For example, if user 'B' dials 8815 on teams client, the dialplan rule normalizes it to +421177228813;ext=8815, i.e full number of a Teams user.
What will change?
As highlighted in above two examples, Microsoft currently ignore extension(if not received), and lookup user with only the main number.
However, Microsoft is making a change to this process. Coming March, Microsoft will only accept the numbers the way it is assigned to your user. For example if Microsoft Phone system receives call to destination number +421177228813, and the user has phone number as +421177228813;ext=8813, the call will straight away fail and Teams will send 404(Not Found) towards SBC.
Microsoft will begin rolling out this change at the end of February and will be complete by mid-March 2023.
How to prepare yourself?
To prevent calls not being matched to the intended user, please ensure that dialed numbers from Teams users and incoming PSTN calls are formatted to exactly match the full phone number configured on the user accounts.
Check that your tenant dial plans include normalization rules that can translate abbreviated dial strings to the full phone number configured on users. In example above, the following phone number normalization rule will translate a dialed 4-digit number to the full phone number configured on the users to ensure calls will succeed:
•Pattern ^(\d{4})$ and Translation +42117722$1;ext=$1
Check that any phone number manipulation/translation rules used on your Direct Routing Session Border Controller are capable of making the translation to the full phone number configured on your users.
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