Cellular carriers are desperate to keep customers, because the cost of marketing and the expenses of churn - a customer signing up and later leaving - are so high that the telcos can afford to give away a lot in exchange for loyalty and regular subscription fees.

Sprint's the latest to offer another twist on current unmetered plans, plans that don't limit your minutes, texting/MMS, or data use. The new "Any Mobile, Anytime" plans allow calls to and from any mobile telephone in the United States without dipping into a pool of minutes. Landline calls still extract minutes, inbound and outbound. A 450-minute plan is $70 per month; 900 minutes costs $90 per month.

All four major U.S. carriers now have $100-per-month subscription offerings that allow unmetered calling to any U.S. telephone numbers. Sprint's fee at that level also includes data and SMS/MMS; Verizon charges $140 per month for the same package.

AT&T has an unmetered voice plan for $100 per month, including for the iPhone, and separately offers no-limit text and data plans for $20 and $30 per month, respectively. T-Mobile includes texting in its $100-per-month unmetered plan; the company has $20 to $30 per month charges for packages of Wi-Fi and 2G or 3G data.

Sprint's offer makes the most sense for younger folk who mostly call others who have cell phones. The $90 plan with 900 minutes saves just $10 per month, but that's still $120 per year if you need some but not a huge amount of outbound landline minutes.

(I use the term unmetered, because all plans have restrictions that prevent them from meeting a definition of unlimited.)