Something similar happened to me: my phone number of 15 years was active on the Sprint cell network via an MVNO. Evidently, when the network was decommissioned, the phone number "disappeared" with it. According to the carrier, it was my responsibility to switch to a new SIM card. The company never notified me of that responsibility, or the deadline. In July, the number stopped working, and I spent maybe 20 fruitless hours on phone support with my carrier, with T-Mobile (since they are the operator of the Sprint network), and with Sprint (which I guess sort of still exists despite being acquired by T-Mobile). The MVNO subreddit had several agents handling this exact problem, claiming they could fix it, but they could not.
I ended up getting one piece of potentially useful advice: after a while, the phone number will possibly get recycled into a generally available pool. IF that happens, and IF I check at the right time, I MAY be able to purchase the number at https://www.numberbarn.com/.
After all that, I just ended up getting a new number. It has now been four months. I check that site a few times a week, no luck yet. I also call the number occasionally, I have a script prepared to try to convince whoever answers to listen to my stupid plea. It's still disconnected.
As for moving forward: just get a new number. You may get the old one back eventually, but don't count on it happening at all, especially not quickly. I had no trouble updating my number for bank accounts. I don't understand why you would be stuck with your old number for a background check service, seems like you should just be able to provide them with a new number.
Maybe that explains why everyone looking at is going "nnnoooope" because the number's owned by someone 3 times removed from existence and those are probably really hard to fix.
I'm curious who the MVNO was, given it clearly wasn't OmniPoint?
The only idea I can think of is to go as far down the *enterprise* rabbithole as possible, if you didn't already try that ( :/ ? ) - in my own experience (with comparatively tiny issues) I've found far less gatekeeping and "wrong number" than the (Interesting™ O.o) phone menus would suggest. Also, one day I also found the enterprise number I'd squirreled away randomly turned into a totally different department (whole new phone menu + tree) but apparently went to the same phones, so don't just try random/wrong phone options, but wrong/unlikely-looking departments' phone numbers too, they might be hubs not spokes. Oh - and calling somewhere Important™ (eg, network operations) at 11:30PM-12AM-1AM might net you a bored, idly curious night person with a bunch of lateral capacity who might not mind the opportunity to do something interesting during their shift lol
I ended up getting one piece of potentially useful advice: after a while, the phone number will possibly get recycled into a generally available pool. IF that happens, and IF I check at the right time, I MAY be able to purchase the number at https://www.numberbarn.com/.
After all that, I just ended up getting a new number. It has now been four months. I check that site a few times a week, no luck yet. I also call the number occasionally, I have a script prepared to try to convince whoever answers to listen to my stupid plea. It's still disconnected.
As for moving forward: just get a new number. You may get the old one back eventually, but don't count on it happening at all, especially not quickly. I had no trouble updating my number for bank accounts. I don't understand why you would be stuck with your old number for a background check service, seems like you should just be able to provide them with a new number.