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MetroPCS(s pcs) has been gunning to be the first out of the gate with a voice-over-LTE (VoLTE) service, and this week it didn’t disappoint. It’s first VoIP handset, the LG Connect 4G, went on sale in Dallas on Tuesday, making it the first carrier to combine its voice, messaging and internet services onto a single all-IP network.

There’s some debate over whether Metro was the first global operator to get to VoLTE. Both SK Telecom and Uplus kicked off VoLTE in Korea on Wednesday, which due to time zones differences was Tuesday in the US. But there’s no doubt that what MetroPCS has accomplished is a milestone for a US mobile industry that has routed its calls over circuit switches since its inception.

The question now is who’s next? MetroPCS had a fire burning under its rear, which drove it to move everything it could to IP as quickly as possible. MetroPCS uses the same spectrum for both its 2G CDMA and 4G LTE networks. That meant every megahertz it devoted to supporting old-school circuit-switched calls was one less megahertz it could use to bolster its much more spectrally efficient LTE network.

Leap Wireless(s leap) faces a similar issue, but it’s been far less aggressive with LTE and has a 3G network to pick up the data slack. Meanwhile, the big operators are deploying their LTE networks over largely untouched airwaves, so they face far less pressure to recycle their old spectrum.

That doesn’t mean, though, they’re not feeling some pressure. By using VoIP, voice no longer becomes a siloed service. Carriers can overlay a voice call with other IP services such as video chat, multimedia sharing and presence. Called Rich Communications Suite (RCS), this communications layer is the operators’ answer to the over-the-top services that are eating away at their voice and SMS revenues. Let’s take a look at each carrier’s stated plans and unstated motivations, one by one:

Verizon: Big Red(s vz)(s vod) may very well be the first Tier I carrier to move to VoIP, not because it needs to, but simply because it can. By year end Verizon will have an LTE footprint covering 230 million people, meaning its customers will be able to receive a 4G signal in most populated areas of the country.

Verizon has already said it plans to launch its first VoIP-powered LTE phones this year, but unlike Metro it’s facing no pressure to shut down to its CDMA networks and harvest their spectrum. Instead Verizon will focus on enhanced VoIP services targeting enterprise customers. According to CTO Tony Melone, in 2013 Verizon’s network will be so widespread it will be able to sell its first phones without CDMA chips.

AT&T: (s t)Ma Bell’s LTE network may not be as extensive as its Verizon’s but it has a technical advantage its archrival can’t claim. As a GSM operator, AT&T can take advantage of circuit-switched fallback, a standard that allows IP calls to revert to circuit-switched calls when customers leave the LTE network. For Verizon, venturing out from under the LTE umbrella means a dropped call (MetroPCS has the same problem).

AT&T has stated it will offer a VoIP-based service in 2013, but AT&T can launch VoLTE anytime it pleases without worrying about whether it has nationwide, or even citywide, coverage. Customers won’t be able to take their enhanced IP services with them onto the 2G and 3G networks, but at least their voice conversations will accompany them.

In an SEC filing last week, AT&T said it plans to sunset its 2G GSM network in five years – a process it has already started in New York. But even with its primary voice network scheduled for retirement, AT&T isn’t facing any increased pressure to move to VoLTE. AT&T’s 3G networks support voice as well.

Sprint: (s s)The country’s No. 3 operator also plans to have VoLTE online in 2013, though it’s actually already running a 2G and 3G VoIP service today in the from of Sprint Direct Connect push-to-talk. Sprint’s LTE network only went live last month so it has a long expansion road ahead before it seriously considers VoLTE. But of all the major carriers it has the most incentive to move voice to its IP networks. Like MetroPCS Sprint is using the same band, PCS, for its 4G, 3G and 2G networks. The faster it gets to VoLTE the sooner it can start shutting down CDMA and refarm that spectrum for LTE.

T-Mobile: T-Mobile is still a year away from launching LTE so you would think it VoLTE would be the last thing on its mind. But T-Mobile is also a very crafty carrier that has shown it can use technology to overcome its limited resources. T-Mobile is deploying LTE on the same band it uses for its HSPA+ network, which after its system-wide reconfiguration will become its primary voice network. If it aggressively pursues VoIP when it launches, it could start shifting its Advanced Wireless Services (AWS) spectrum from HSPA+ to LTE, which would in turn clear up more room for LTE.

Feature image courtesy of Shutterstock user Shawn Hempel