It wasn’t very long ago when carriers started offering unlimited voice plans for $100 a month, but a slew of regional players and MVNOs over the last year, have been putting additional price pressure on the four major carriers to discount rates even more. Recently, Boost Mobile, the MVNO running on Sprint’s smaller Nextel network, started offering a $50 plan that includes unlimited voice, texting, Web access and push-to-talk services. That matches other regional players such as MetroPCS and Leap, which offer plans for $45 to $50, but are limited geographically. Virgin Mobile USA (NYSE: VM), which runs on the Sprint (NYSE: S) network, offers an $80 unlimited plan. RCR points out that T-Mobile is pairing its $50 unlimited voice plan with a $35 unlimited text and data plan for a total of $85 a month, which matches a similar data plan being offered for the T-Mobile G1.

It’s hard to understand why T-Mobile is headed in this direction because it seems like it would cannibalize revenues from two of its flagship services: the company’s HotSpot@Home service, which offers unlimited calling over Wi-Fi, and its MyFaves program, which offers unlimited calling to five of your favorite people. Each costs more money, but often requires a minimum voice plan of about $50 to qualify. Why would customers sign up for either service if they had an unlimited voice plan for $50 a month? Perhaps, T-Mobile has retention problems. In Q4, it said churn jumped to 2.4 percent, among contract holders, from 1.8 percent in the same period a year earlier. One reason for the higher churn? “Competitive intensity,” the company said. Sort of gives new meaning to the company’s slogan: “Stick Together.”