The operator didn't disclose how many devices it has sold, so it's not clear that this is evidence of overwhelming demand for the G1, the first phone to run Google's Android software.
The news follows reports last week that some customers trying to preorder a G1 were told there were none left. At the time, T-Mobile said there were still a limited number available.
T-Mobile now says it tripled the number of phones it had earmarked for presales of the G1 and that customers have ordered them all.
That doesn't mean people who want one are out of luck, however. People will be able to visit a store to buy a G1 in person when it launches. In addition, customers can still preorder a device through Oct. 21, but it may not arrive until a later, unspecified date.
At the announcement of the G1 two weeks ago, many analysts warned against expecting initial sales of the G1 to compare to initial sales of the iPhone. But if anticipation is as high as T-Mobile says it is, the phone could help the operator prevent customers from switching to AT&T to get Apple's iPhone, something one analyst group said has been happening.
The NPD Group on Monday said it found that 30 percent of U.S. consumers who bought the iPhone 3G between June and August switched to AT&T, the exclusive operator for the iPhone, from another carrier. That's more than the average volume of phone users switching carriers, which is 23 percent, NPD said.
Verizon appears to have been hit the worst, according to NPD. Of new AT&T iPhone customers during that period, 47 percent came from Verizon, NPD said. Just 19 percent switched from Sprint and 24 percent came from T-Mobile.
According to NPD, the iPhone 3G was the number-one smartphone based on unit sales from June to August. The BlackBerry Curve was number two, followed by the BlackBerry Pearl.
The iPhone has also driven down smartphone prices, NPD said. The average price of a smartphone sold between June and August was US$174, down from $236 during the same time last year, NPD said. The G1 costs $179 with a contract.