It can be more complicated with US carriers. Verizon and Sprint (the 1st and 4th biggest carriers) traditionally have used CDMA technologies without SIM cards.

It can be more complicated with US carriers. Verizon and Sprint (the 1st and 4th biggest carriers) traditionally have used CDMA technologies without SIM cards. This is changing with LTE, but their non-LTE technologies do not use SIM cards. Furthermore, these carriers will generally refuse to activate a phone from another carrier or retailer to be used on their network. So, while you can just put a Verizon SIM in a tablet for LTE data, you can't do the same with a phone. (I'm not sure how this will work with VoLTE.)