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For nearly two years, FreedomPop has offered budget-minded mobile consumers a compelling proposition: a bare-bones voice and data plan at absolutely no charge. The public reacted well to the idea of free, leading more than 500,000 customers to sign up for service with the LA-based virtual operator.

But FreedomPop believes that some of its customers are willing to pay good money for an optimized service. Starting today, the company is offering a premium voice service that it claims will deliver better audio quality and ensure customers can make and receive phone calls at times and in places where their voice service is normally spotty.

FreedomPop hasn't developed some new voice technology or network. Rather, it's falling back on 2G, the voice technology that powers the vast majority of mobile phone calls today. One of the major reasons FreedomPop has been able to offer free voice services is that it has bypassed mobile carriers' traditional voice networks and routed its phone calls as IP sessions over 3G, 4G and Wi-Fi data networks.

The only problem with that approach is that mobile data connections aren't always the most reliable – as anyone who has tried to make a Skype call in crappy 4G coverage can attest to. That problem is particularly acute for FreedomPop. As a mobile virtual network operator (MVNO), FreedomPop buys network access wholesale from Sprint, which trails all U.S. carriers in 4G speeds and coverage.

Customers getting a gratis 200-minute voice plan probably have little to complain about, but according to FreedomPop CEO Stephen Stokols, the virtual carrier is starting to attract a higher class of customers -- notably small business owners -- who want a higher class of service, and they're willing to pay for it.

So FreedomPop has started to buy voice minutes off of Sprint's CDMA network, and it has developed new software for its Android phones that will detect the quality of the data connection available. If network speeds fall below 400 kbps, FreedomPop will reroute calls over Sprint's 2G network.

The service will cost $2.99 a month, and it will apply to whatever plan a FreedomPop customer subscribes to. Thus, if you're on the free plan, your 200 voice minutes can be routed over either the 2G or VoIP network. If you buy additional voice minutes from FreedomPop (for instance, its $20 unlimited voice plan), all of those calls will be 2G eligible as well.

What's more, FreedomPop will give all of its customers a free month's access to the new 2G voice feature. "We want to give everyone a taste of it," Stokols said.

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