T-Mobile USA's $4 billion network modernization is off to a running a start.  

In an update on its progress since the initiative was announced three months ago, T-Mobile ticked off a number of important milestones on the way to upgrading its network.  

First and foremost, the company announced in May that it had signed multi-year contracts with Ericsson and Nokia Siemens Networks (NSN) to provide the equipment necessary for the network upgrade.  

NSN and Ericsson are providing equipment compatible with LTE-Advanced Release 10 at 37,000 cell sites. To that end, T-Mobile said that by the end of this month it expects new equipment to be installed at the first 400 modernized GSM/HSPA+ sites, and it plans to grow that number to more than 2,500 sites by the end of July. 

The LTE-Advanced trials are set to begin this summer with broad LTE deployment on track for 2013.

Dave Mayo, senior vice president of technology for T-Mobile USA, said in a blog on the carrier's website that the company has secured zoning approval for more than19,500 sites and have completed necessary entitlement work, including obtaining building permits for more than 4,000 sites.

T-Mobile is also turning on small scale tests of HSPA+ in its 1900 MHz spectrum. In fact, the company recently confirmed that it had deployed such a test at Moscone West conference center in San Francisco, the site of both Apple and Google's developer conferences. 

"Some customers with unlocked iPhones recently saw evidence of those tests; clocking much faster data speeds at the Moscone West conference center in San Francisco," Mayo wrote. "More of these speed sightings will occur as we work toward introducing 4G HSPA+ service in our 1900 MHz spectrum in a large number of markets later this year."

Without actually offering the iPhone as part of its portfolio, T-Mobile is hoping to attract customers with unlocked iPhones. T-Mobile estimates that it already hosts more than 1 million iPhones on its network.