This is an official response from @IUSACELL on Twitter when I questioned them on why they had blocked the verification number in the UK:
No es por parte de Iusacell es general en todas las compañías, Apple está registrando actualización en sus aplicaciones y se presenta este inconveniente. EB
(Translation: (The problem) is not from Iusacell's end, it's happening to all companies, Apple is updating its applications and this is the cause of the inconvenience. EB)
And upon asking on an estimate for Apple completing its upgrades:
No dio tiempo estimado, una disculpa por la molestia que esto te ocasiona, que tengas buena tarde. EB
(Translation: They did not give us an estimate, we apologize for the inconvenience this may cause you, have a good evenin. EB)
It's POSSIBLE that Apple has been updating/upgrading/fixing the system on their end due to an inordinately high-demand compared to their original preparations and projections, but, it's also been nearly two months or more since I initially started having problems both with iCloud backups and iMessage verification. You would think Apple would have already fixed this, if the problem is in fact on their end...
I have to go to Iusacell in a week's time and I'll be consulting with an executive that I've had a 100% success rate in problem solving with Iusacell in the past. I'll see what she has to say about this matter, and/or if she can find a solution to it, because it's getting old to have to use my iCloud account for iMessage. I know it's a seemingly small inconvenience, but it's not, most people have my number, but not my iCloud account, and I would rather not go about handing it out.