All you n00bs were in school or something when RedPhone dropped in 2012-13. It was a huge innovation for solving Sybil attacks on IM systems.
And then a couple of years later sim cloning became easy as the cryptography is (still) circa 1996.
Solving this problem is one of the many use cases we see for Indranet. Based on Schnorr and ECDH. Self sovereign keys, and p2p metered relay service, cost of attack is linear with the targets.