As a result, he predicted T-Mobile will displace Sprint as the nation's third-largest carrier, bumping Sprint to fourth. Sprint said in May that it has 57.1 million customers, compared to the 56.8 million for T-Mobile at that time.
"I predict this will be the time they [Sprint] go down" to fourth place, once the quarterly numbers are finalized, Legere said in a conference call. T-Mobile's market capitalization is already double that of Sprint's, he added.
T-Mobile's total for the quarter will be 58.9 million postpaid, prepaid and wholesale customers, Legere predicted. That's an increase of 41% over the same quarter a year ago.
T-Mobile has added more than 1 million customers each quarter for the past nine quarters, he added. The surge comes over a two-year period when other carriers stayed largely flat or lost customers.
If T-Mobile jumps to third in terms of subscribers, it would be behind both AT&T and Verizon Wireless.
Sprint declined to comment Thursday on its current subscriber numbers. The carrier hasn't announced the date for its next quarterly earnings report, which is expected in August.
"While the media is going to focus on who is three or four, customers don't care," said Sprint spokesman Scott Sloat in an email. "Sprint is focused on long-term growth by offering customers value, providing a great experience and having a reliable and consistent network. With new offers like Sprint's All-in and a network that third parties increasingly praise, we continue to make progress."
Sprint committed a momentary marketing blunder with its All-in plan, which initially imposed a 600 Kbps limit on streaming video. But CEO Marcelo Claure stepped in to quickly remove the limit later the same day of the announcement.
Roger Entner, an analyst at Recon Analytics, said the preliminary numbers from T-Mobile are "excellent," adding, "It will be very difficult, but not impossible, for Sprint to get the numbers to stay number 3."
Entner has also said that customers don't pick a carrier based on its ranking. Instead, customers pick based on pricing, wireless coverage and network quality, primarily.