The telco is in charge of what SIM is mapped to a given number. There's not anything technical the customer can do there; access control is up to the telco.

The telco also needs a process to reclaim the number when you stop paying for it.

Often SIM swaps are done via porting the number to a different telco, and telcos are compelled to do ports in many jurisdictions.

If you're really worried about it, I guess you could look into what it takes to get a number block assigned to you as a competitive telco. That would likely be hard to get transfered out from under you, but the effort may not be worth it.

Also, if the price to get a sim swap is $1k (plus a % of ill gotten gains!), that's high enough to keep out untargetted attacks, IMHO, which isn't a terrible place to be.