T-Mobile will maintain MetroPCS’s VoLTE service, but its future is up in the air

On Wednesday we raised some questions on whether T-Mobile USA would continue MetroPCS’s(s pcs) aggressive voice-over-LTE (VoLTE) rollout if the companies’ successfully complete their merger. It turns out the answer is both yes and no.

In an interview with GigaOM, T-Mobile CTO Neville Ray said that T-Mobile would continue supporting Metro’s VoLTE handsets and mobile VoIP service, but whether it would expand that service to its own network and handsets was still an open question. Ray stressed that T-Mobile eventually plans to migrate its voice traffic to its all-IP 4G networks, but the big issue is timing.

“VoLTE will come,” Ray said, but he pointed out it’s a relatively unproven technology that will take several years to mature into a commercially viable service. “We will certainly support the VoLTE services that MetroPCS has today,” he said. “But is that the VoLTE we want to populate the new network with? TBD.”

In its initial manifestation, VoLTE probably won’t have much of an impact on consumers. Carriers will transfer their voice services from one network to another and likely charge the same rates for the service. But as VoLTE evolves, carriers will be able to offer a range of multimedia and IP communications features. They will also be able to implement technical enhancements such as HD voice (subscription required) that they simply can’t support on circuit-switched voice networks today. For carriers, there is an even larger implication in VoLTE. Once they turn off their old voice network, they can phase out their 2G and 3G systems are repurpose that spectrum for LTE.

T-Mobile’s VoLTE decision will be based on both demand and logistics, Ray said. MetroPCS faces big capacity constraints — every megahertz of spectrum it devotes to 2G voice is one less megahertz it has to fuel its bandwidth constrained 4G mobile broadband network. T-Mobile, however, has a lot more breathing room in its airwaves. “The rush to get to VoLTE is less for T-Mobile because we have such underlying strength on GSM and HSPA+,” Ray said.

The companies expect the deal to close in the first half of 2013, but T-Mobile’s own LTE network won’t go online until the latter half of the year. You can’t offer a VoLTE service with the LTE component. If it added VoLTE clients to the new HSPA+ handsets it plans to sell existing Metro customers, it would still be sometime before they could access a nationwide LTE network.

In addition, T-Mobile isn’t planning to integrate Metro’s LTE systems into its own. Instead, it plans to shut down Metro’s radio and core networks as soon as the last CDMA phone goes offline. Ray said T-Mobile will keep about 1000 cell sites, and 6000 antenna nodes, but otherwise it will replace all of Metro’s gear with its own base stations and core. That includes the service delivery architecture – in telco parlance is called IP Multimedia System or IMS – that hosts Metro’s VoLTE applications.

If T-Mobile decides to hold off on VoLTE, what we’ll likely see is a large-scale phasing out of VoLTE handsets as MetroPCS customers switch to HSPA handsets (Considering nearly two-thirds of MetroPCS customers switch out phones every year, that migration could happen quite quickly). That phase out would then be followed several years later by a gradual reintroduction of VoLTE, after T-Mobile’s nationwide LTE network goes online and the VoIP technology has cut its teeth in the market.

But Ray held out hope for another possibility. Depending on the states of the VoLTE technology and T-Mobile’s network, the carrier could jump into VoLTE whole hog as soon as the merger closes. That would mean the new T-Metro would start VoLTE-HSPA hands shortly after the closing, utilizing the VoIP capabilities of Metro’s 4G footprint initially and expanding to T-Mobile’s new LTE network as it comes online.

There are few technical barriers standing in T-Mobile’s way, Ray said. Both MetroPCS and T-Mobile share the same IMS core architecture, supplied by Mavenir Systems (see disclosure). Even when the T-Mobile shuts down the Metro network, VoLTE handsets could theoretical transition straight onto the T-Mobile core. The question T-Mobile has to weigh, Ray said, is whether it sees the need to launch a large-scale VoLTE service so soon.

Disclosure: Mavenir Systems is backed by Alloy Ventures, which also backs GigaOmni Media, the parent company of GigaOM. Alloy’s Ammar Hanafi is on the board of both companies.

Tower Image courtesy of Flickr user Nikhil Verma