T-Mobile pushed the envelope of unlimited calling plans this morning by announcing the cheapest unlimited family plan. Once a customer has a single $99.99 plan, he or she can add lines to the plan for an additional $49.99 per month, and that includes unlimited calling, SMS, MMS, and IM.

It’s a pretty incredible deal, considering that the previous family plan pricing started at $59.99 for two lines and 700 minutes, and then customers would have to add on their own messaging plans too.

Let’s say you need cell phones for you, your spouse, and your 2.5 kids. That’s a total of five lines. With T-Mobile’s new plan, you’re paying $249.95 for unlimited messaging and calling. AT&T charges $199.99 for the first two lines, and each additional line is $9.99 (max of 5). For the same family, that’s $229.96 for unlimited calling only. But unlimited messaging will cost you $20 per line. That’s a $100 more per month.

Sprint doesn’t offer an unlimited family plan, but Verizon Wireless has three various packages. The Select package is most similar to T-Mobile’s, with unlimited calling and messaging, but it also includes unlimited mobile Web access for $529.95.

Overall, T-Mobile’s deal sounds like a steal if you’ve got a house full of talkative teenagers. Save the extra dough for college tuition, or your midlife crisis.