Credit: Joe Maring / Android AuthorityRecently, we learned about a secret T-Mobile plan that costs just $25 per month, significantly undercutting every major US carrier. Before you rush out and sign up, however, you’ll want to make sure you understand exactly what you’re getting.On paper, the offer sounds compelling and, in a vacuum, it really is. That said, there are a few caveats you should consider before making up your mind.