It's already been a big week for cellular network upgrades in the U.S., and now analysts say that T-Mobile US Inc. could soon be ready to join the fun by rolling out fixed/mobile convergence services in the fall.

Third-ranked operator Sprint Corp. (NYSE: S) said on Tuesday it will launch a mobile WiMax network due to cover millions of subscribers by the end of 2008. (See Sprint Goes WiMax.) Meanwhile, Verizon Wireless , the second largest operator, laid out its concept of Advanced-IMS -- an upgrade to the fixed/mobile convergence standard that the carrier reckons will allow it to support IPTV, video, and multimedia services over wireless networks via an IP Multimedia Subsystem core.

It seems, however, that the smallest of the Big 4 U.S. cellular operators might be the first to enable users to jump between cellular and WiFi networks on their mobile phones. T-Mobile could have FMC services that use the unlicensed mobile access (UMA) specification ready as early as the fall, according to Roger Entner, VP of Wireless Telecoms at analyst firm Ovum Ltd.

"They have basically finished the upgrades to their APs, they're waiting for the handsets," Entner tells Unstrung. "They could launch in the Fall."

"It's clear that T-Mobile US is gearing up for a launch soon," agrees Steve Shaw, director of marketing at UMA pioneer Kineto Wireless Inc. , in an email.

The carrier itself isn't giving any specifics yet but confirms its interest in the technology. "T-Mobile is interested in the replacement or displacement of landline minutes," a spokesperson writes in an email reply to questions. "We believe the future will be about leveraging diverse forms of radio access technology for our customers and Unlicensed Mobile Access, we think, is one of the technologies that will help us continue to deliver on that promise."

T-Mobile's network of public access WiFi hotspots may actually give it an advantage over other carriers in this instance. T-Mobile says it now has 7,836 locations in the U.S. Unlike many of its cellular rivals, the operator largely owns its WiFi network, rather than relying on partnership deals. This may make upgrades like FMC easier for the operator to install.

UMA convergence technology is popular with many operators and network vendors because it allows carriers to maintain central control over calls passed between WiFi and cellular networks. Opinions are mixed, however, on how useful the technology will be for enterprise users. (See UMA : Next Year's Model? and DiVitas Grabs VC Cash.)

— Dan Jones, Site Editor, Unstrung