Authority for the North American Numbering plan currently falls to the FCC, after the breakup of AT&T; they contract out administration, Ericsson got a 5 year contract in 2017, previously it was administered by Neustar (and Lockheed Martin, before Neustar was spun out).

I'd imagine Ericsson would spin up a new CA instance just for this. As the administratior, they keep the records of who phone numbers are assigned to, and where they've been ported.

My understanding is once a +1 area code is allocated to a country, that country's telecoms regulator manages further allocations (or contracts it out).