You could totally do that if you want. It is sometimes cheaper to go that route. If you do that though, there is a slight waiting period before you can sign-up for another line on AT&T. Typically a 6-month period. Then again, if you weren't going to sign-up for AT&T in the next 6 months anyway, then that's not an issue.
You do need to wait past the 14-day trial period before leaving AT&T. Which means you'll end up paying for part of a month of service. So the savings may not be so significant in the end (which was the point of raising the ETF, because people did this exact thing).