> if you port your number from Operator A to Operator B, your number prefix still sits in A's range, and calls are signalled to Operator A. They can then tell you to try Operator B. B may then need to tell you the user is roaming and how to reach them. But yes, current firewlls leave a lot to be desired!

Not in all networks as far as I'm aware. UK is an annoying example of not having a central database of ported numbers (with ACQ), where a redirect is setup in the old network. I once ported my number in the UK and had huge issues receiving international calls or 2FA codes, it took me ages to work out and only got it sorted by leaving the number entirely and getting a new one.

Otherwise, like here in Germany, it's done with a proper database and the call never passes through the old network.