T-Mobile's (TMUS 0.43%) been on a tear recently. The company's Un-carrier moves have led to impressive customer gains. The company's success, however, hasn't gone without a response from the competition. AT&T (T 1.39%) responded last week to T-Mobile's customer acquisition success with $450 in credits. But this week T-Mobile has jumped in with its own incentives for customers to switch to its network. As the carrier war for customers rages on, companies like Apple (AAPL 0.81%), whose costly smartphones are highly dependent on subsidies, financing, and credits, will likely benefit.

T-Mobile's planned assault
T-Mobile has generated more than 1.6 million total customer additions and has reported positive branded postpaid net customer additions for three quarters in a row. Its success in acquiring customers follows T-Mobile's implementation of an aggressive plan to disrupt the industry. It's referring to the plan as Un-carrier -- and it's working.

"Our Un-carrier moves have clearly upended this industry," T-Mobile CEO John Legere said in a press release yesterday that highlighted preliminary fourth-quarter results. "Over the past 12 months, 4.4 million customers have come to T-Mobile in response to greater flexibility and choice. We have clearly struck a chord with customers and will continue to look for ways to expand on that in 2014."

T-Mobile has rolled out its Un-carrier initiative in four phases so far, beginning in March 2013.

  1. Introduced "Un-Carrier" plans that said goodbye to subsidies, replacing them with interest-free financing.
  2. Introduced a "Jump" program that allows subscribers to upgrade their phones at smaller intervals.
  3. Added international texting and 2G data in 100 countries to its Un-Carrier smartphone plans.
  4. Offered credits to customers switching from the three major national carriers.

The latest battle
The fourth phase of T-Mobile's Un-carrier moves, announced yesterday, looks like a response to AT&T's just-announced credits. T-Mobile's move offers credits to switching customers from AT&T, Sprint, and Verizon. But as The Wall Street Journal author Ryan Knutson said after AT&T announced its $450 of credits, T-Mobile was already widely expected to announce a similar promotion before AT&T's was announced. So who knows which carrier was planning to offer credits to switching customers first?

T-Mobile's credits are irrefutably more aggressive than AT&T's. They offer up to $350 in early termination fees per line (up to five lines per family) and up to $300 in credits for phone trade-ins. That's potentially $650 in credits per line for customers switching from any of the three major national carriers.

iPhone 5s.

Time for a new iPhone?
Since T-Mobile's announcement detailing the move said the deal would only be available at "participating T-Mobile location[s]," I called around to see just how prevalent the deal was. Every location I called was well aware of the deal.

The intense competition between carriers is great news for Apple investors. As long as they are fiercely competing for customers, the subsidies, interest-free financing, and credits that help drive iPhone sales will remain intact.