The Apple SIM — a programmable SIM card that lets you choose carriers after you purchase an iPad — was hailed by many as the beginning of the end for the SIM card. But some US carriers are throwing a wrench into the new system, and none other than outspoken T-Mobile CEO John Legere has taken to a lengthy "tweetstorm" to try and clarify the matter.

The first and second largest carriers in the US, Verizon and AT&T, aren't playing ball with the Apple SIM. Verizon has opted to exclude itself from the system entirely, while customers who select AT&T as their carrier of choice will have to buy another SIM card if they wish to switch carriers again.

That's not the only way the Apple SIM isn't working as planned: there's also a bit of confusion when buying an iPad from a carrier store. As Legere explains, those iPads, depending on which carrier you shop at, will either have that carrier's SIM already installed, or it will have an Apple SIM pre-programmed to that carrier.

In all, it's certainly not quite the future promised by the Apple SIM, but the concept is still in its infancy — hopefully carriers will get on board sooner rather than later.