There are some good details in the linked location below as to the various PLMN's being deployed and what they represent.
It seems that plausibly (credit Jamal Baker) that::
312-250 = Harmonized Sprint equipment in areas with otherwise poor TMobile towers, to be eventually converted to T-Mobile equipment
312-490 = Towers to eventually be removed, sold to Dish, or converted to all N41.
Truth be told I did eventually find it late last night and was going to update my post here.
I appreciate you reconfirming where this bugger is located.
It's a subtle thing and I wonder how many people even know this iOS feature exists. At least on Sprint is was fairly straight forward as a selectable free feature "Calls on iCloud Devices" if memory serves me correctly.
All the best,
It's really buried down in there. When first you log in, on the top right corner click on "My account," then "Profile". At the bottom of the next screen, you should see "Multiple Devices." Click that, and on the next screen make sure that DIGITS is enabled. Here's what mine looks like.