Cell-phone monthly service costs could be on the way down.

TheStreet.com has learned that T-Mobile ( DT) may soon be announcing a reduced price "everything" that which could force all the other cellular carriers to follow suit.

T-Mobile has been testing a $50-per-month unlimited voice plan in the San Francisco area. It's been pretty successful, so much so that T-Mobile is thinking about rolling the plan out nationwide. When you add TM's $35-per-month unlimited data/text plan you get a combined $85-per-month unlimited, all-you-can-use plan for only $85.

That beats rivals Verizon Wireless ( VZ), AT&T ( T) and Sprint ( S), which all have their own "everything" plans that start at $99 per month. That's a difference of at least $168 per year.

There are a number of other companies, very small wireless carriers, that are actually offering even lower prices. Boost Mobile, the pay-as-you-go division of Sprint, is currently offering a $50 voice/data/text monthly "everything" plan. Newcomer MetroPCS ( PCS) is currently trying to make a big splash in the New York City metro market.

Dallas-based MetroPCS is cutting everyone's price and charging only $40-per-month for its voice/data/text plan, but it's in the process of building out its nationwide network, so coverage is somewhat spotty. A quick check of Metro's New York City-area map shows a distinct lack of coverage once you leave the five boroughs.

T-Mobile hasn't exactly formally announced this new lower pricing. All it is officially saying is that tests in San Francisco are under way and that this new plan is also being offered to select T-Mobile customers. According to reports, loyal customers are those who have remained under contract with T-Mobile for 22 months or longer. It seems like a clever way to keep these customers once their two-year contracts are up for renewal.

Past that, T-Mobile doesn't have much else to say about all this -- at least for now. Expect the other major carriers to follow suit and lower their prices. too.
Gary Krakow is TheStreet.com's senior technology correspondent.