This was discussed in the specs of the latest version (v3), and actually Apple and other GSMA members stood in the way of this. AT&T and Verizon wanted to include lock as part of the specs but it didn't pass and they were accused of collusion: https://www.macrumors.com/2018/04/20/apple-verizon-att-switc... So in summary, it's the opposite of a carrier lock.