"operators of the SIM cards likely run their own Mobile Virtual Network Operator (MVNO), which is essentially a telecom company piggy backing off of the infrast

"operators of the SIM cards likely run their own Mobile Virtual Network Operator (MVNO), which is essentially a telecom company piggy backing off of the infrastructure of a more established network." This is certainly the trick, it may have little to do with the SIM card. (personal opinion of a retired Telecom R&D engineer)