With T-Mobile's uncarrier plans no longer a secret, the phone provider is currently engaged in a battle for customers as it tries to draw them away from competitors like Verizon and Sprint. One of the big products that most analysts think T-Mobile needs in order to succeed is Apple's iPhone, and the carrier just started selling the iPhone 5 on Friday.

More than simply offering the iPhone 5 for sale, T-Mobile has decided to engage in an extremely generous trade-in program for those interested in moving from their current carriers. Trade in an iPhone 4, 4S or an iPhone 5 and you knock the $99 cost to zero.

So how is T-Mobile's plan doing after the first day? According to its Chief Marketing Officer Mike Sievert, pretty darn well.

"Today has been gangbusters for T-Mobile," Sievert said in a statement to AllThingsD. "We experienced lines out the door this morning at nearly all of our almost 3,000 stores nationwide."

Obviously the big draw is the iPhone 5, and Sievert didn't make any indication otherwise.

"Clearly they want the iPhone 5, and they are voting with their feet that they want it from T-Mobile," he said.

Without the trade-in offer, the T-Mobile iPhone 5 costs $99 up front, and $20 a month afterwards.

Considering the iPhone 5 is already months old (in smartphone years, that makes it middle-aged, even though people act like it's older than that, too), it's pretty impressive that T-Mobile was able convince people to line up for it.

No hard numbers were given to demonstrate T-Mobile's success on launch day, but the true test will be whether the company can sustain any momentum it gained. Some observers claimed the excitement withered away as the day wore on, so we might need to see some data before any more predictions can be made.