Unity does not get the name from call manager, just the number. On external calls from non-unity subscribers it uses the unity server mailbox to leave the voicemail in the users mailbox.

For calls from extensions that are on the unity server, unity sees that extension as a subscriber so uses that users mailbox to send the VM. This is why you see a name on these, because it knows it is from the number. Example, if Bob at extension 3000 calls Joe at extension 3010, when the call rolls to VM unity see's it is 3000 calling 3010 and has a subscriber with extension 3000 and another with extension 3010 so it can populate both names.

There is a seperate subject line customization for external calls, then internal calls so you can make the external calls from non-subscribers subject different.