The multi-carrier Apple SIM in the iPad Air 2 and iPad Mini 3 lets US customers purchase data from either AT&T, Sprint, or T-Mobile. But it turns out that if you want the option of switching carriers, you'll either have to avoid AT&T or acquire another SIM card.

When you set up cellular data on a new iPad with the multi-carrier SIM, you'll get this screen, which lets you choose a carrier:

Click on AT&T and you'll see this warning, stating that once the SIM is activated on AT&T's network, it will be locked to AT&T and that you'll need a new Apple SIM to change carriers:

Carrier locking is not a problem when you use the Apple SIM to purchase Sprint or T-Mobile service. Instead of showing a warning, clicking on Sprint takes you directly to purchasing options:

Clicking on T-Mobile brings a similar result:

Apple public relations pointed Ars to a support FAQ that confirms the AT&T locking.

"Using Apple SIM, you can choose from different cellular carriers and their various programs. The data plans vary by carrier. For instance, in the United States, you can choose a domestic plan from either Sprint or T-Mobile and also pick an alternate plan from the other carrier as needed. When you choose AT&T on iPad Air 2 and iPad mini 3, AT&T dedicates Apple SIM to their network only," Apple wrote. "If your Apple SIM becomes dedicated to a specific network and you want to choose from other carrier programs, you can purchase a new Apple SIM from an Apple Retail store."

An Apple customer service representative told Ars that Apple SIMs are available for free at Apple retail stores. (UPDATE: I went to an Apple retail store and they charged me $5 for a new Apple SIM.)

T-Mobile US CEO John Legere blasted AT&T for locking the SIM:

@WaltBTIG iPads bought in Apple Store with apple sim, if activated on @ATT it locks, if @TMobile it doesn't..it stays flexible. #keepflex — John Legere (@JohnLegere) October 24, 2014

AT&T confirmed to Ars that customers who initially select AT&T on Apple's multi-carrier SIM will have to replace the SIM to switch to a different carrier but did not explain why the SIM works differently with AT&T than with Sprint and T-Mobile.

Sprint declined comment.

The Apple SIM is removable with a paperclip-like tool that comes in the iPad box (a regular paperclip will work fine, too).

It should be easy to get a SIM from a different carrier if you want to switch later. This would be no different from buying an unlocked iPad directly from a carrier and then swapping in a different SIM later on. The Apple SIM is still more flexible than SIMs that come directly from carriers, but it's too bad it isn't quite as flexible as it could be. On the plus side, new Apple SIMs are available if you make a trip to an Apple retail store.

Verizon is not available on the multi-carrier SIM, so you'd have to buy the device directly from Verizon or swap in a Verizon SIM if you want its service.