when an External User makes call to the internal user:

Step 1: A PSTN User dials the number 600-234-3465. This number is assigned to a user ocsa@ocsdom.local as a primary phone number.

Step 2: Call reaches to the PSTN and then to the local PBX of the ocsa@ocsdom.local.

Step 3: If Mediation Server is placed then the Basic gateway forwards that call to the Mediation Server. Otherwise the Advanced Media Gateway forwards the call to the Director. Mediation Server normalizes the number to E.164 format before forwarding the call to the Director. Mediation server normalizes the number as per the location profile assigned to it.

Step 4: Director performs Reverse Name Lookup. Now Director knows that +16002343465 belongs to ocsa@ocsdom.local. Director finds out the Home Server of ocsa@ocsdom.local and routes the call to the OCS Server.

Step 5: OCS Server checks if ocsa@ocsdom.local has any call forwarding rule. If found then OCS Server forwards the call accordingly. If not then finds out active end points of the user and forks the call to every active end points where the user is signed in.

Step 6: Client determines if the user should be notified of the call based on presence status.

Step 7: If yes, ringing toast is displayed at active end points (where ocsa@ocsdom.local has logged in).

Related Links:

What is Reverse Name Lookup (RNL)?

Normalization of phone number at the client end or server end?