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All you can eat. I remember how sweet those words sounded when I first heard them in connection with something other than food: a flat-rate dial-up Internet account back in the 1990s. Recently all four major national cell phone carriers went the same route for voice calls, announcing new plans that set no limits on talk time. But unlimited talk is pricey (the plans charge $100 a month), and several other drawbacks make the offers less appetizing than they sound.

For starters, the plans don't do much for families. Verizon Wireless, which was the first to announce its unlimited-calling plan, gives no break whatsoever for additional lines: Each one will cost another $100. Same nondeal with AT&T Wireless & T-Mobile.

Sprint does offer discounts, starting at $5 off for a second line and increasing the reduction by another $5 for each additional line, up to five in all. But that totals a still-hefty $450 for a family that needs five phones.

Landline Substitute?

The pricing strategy makes substituting cell phones for landlines a nonstarter for households with more than one person, since you don't pay a dime more for a landline no matter how many family members (or friends) use it to talk their hearts out.

Also, substituting a cell phone for a landline introduces problems such as getting agencies to respond to the right location for emergency 911 calls, or making sure the handset is charged.

What about overseas calls? You can easily find deals on long-distance landline services, but international calls get very expensive on cell phones. Also, if your broadband service is tied to a landline (DSL or ISDN, for example), and you don't have a good alternative, you have to stick with the landline anyway.

Only Sprint Includes Data

Another reason I'm not impressed by unlimited talk plans: Most don't help with fees for data service, which are an increasingly big chunk of my cell phone bill. AT&T and Verizon charge separately for messaging and data services; T-Mobile includes unlimited messaging (instant, text, and picture and video), but not data.

Only Sprint's appropriately named Simply Everything plan bundles unlimited voice, data, and messaging, which makes it the best deal. Sprint even throws in GPS navigation service. Sprint's offer comes with a lot of fine print, however: For example, the company can terminate service if most of your voice minutes or data use involves roaming, and you can't use your phone as a modem.

Of course, you still have to pay regulatory fees and taxes, too, so all these plans top the $100 mark by the time the bill arrives. For really big talkers--mobile professionals, for example--they'll be worthwhile, eliminating worry about whether you're using night, weekend, rollover, or regular daytime minutes.

But cell phone talk minutes have become so cheap that most other people probably wouldn't benefit. My husband and I rarely get close to using the 450 shared anytime talk minutes in our low-end AT&T plan (at this writing we have nearly 4000 rollover minutes). And even with data fees, we don't pay anything close to the $200 we'd be charged for one of the new plans.

The bottom line: I'm not thrilled about these new offers. People who live on their cell phones will want to investigate them--especially the Sprint offering--but unlimited nationwide voice calling is probably overkill for most folks.