Sprint, which is trying hard to catch up with its larger rivals, has an offer for families looking for unlimited data that\u2019s seems too good to be true. And what you know, it is too good to be true.\nThe company\u2019s offer for up to five lines of unlimited voice, texts\u00a0 and data for $90 (plus the usual taxes and fees) is good for only one year. By \u201cgood for one year\u201d I don\u2019t mean that you have a year to sign up. I mean that the big discount Sprint is offering this month expires at the end of March, 2018, and pricing reverts to the much more expensive standard rate, an increase that's more than double.\n\n[ The votes are in: Which mobile data provider is best? ]\n\nI\u2019m not saying Sprint doesn\u2019t disclose this. It does. But you\u2019ve got to look in the footnotes to understand the plan has some serious gotchas.\nHere\u2019s how Sprint describes the offer.\n\u201cUnlimited Data, Talk and Text for $50\/month for the first line with AutoPay, two lines for just $90\/month; and the third, fourth and fifth lines are FREE. A family of five will pay $90\/month for five lines.\u201d\nThat, in fact, is a good deal, and significantly cheaper than offers for that many lines from the three other major carriers. But after March of 2018, that family of five will be paying $190 a month for the same service.\nWhen the promotional offer expires, the cost for a single line goes up $10 to $60 a month; service for two lines will also go up $10 to $100 a month; and each additional line, up to three, will cost $30 a month.\n\n[ How to save on mobile plans: Your guide to 16 no-contract carriers ]\n\nTo be fair, I should mention a point that a Sprint service representative who walked me through the plans made. \u201cOver the course of the year, your family would save $1,000,\u201d he said. Actually, $1,080 over 12 months, and he\u2019s right: That\u2019s a real savings.\nYou could, of course, sign up for a year and then jump to another carrier, but if you\u2019ve purchased a phone from Sprint on an installment plan, as most consumers do these days, you\u2019d have to pay off the remaining price of the phone.\nThere are a few other caveats to consider. Unless you sign up for autopay and e-billing, each line on the discount plan will cost an additional $5 a month. Streaming video is capped at 480 pixels, a resolution that\u2019s fine on a phone, but not really adequate if you stream from the phone to a television. If you want HD quality, you\u2019ll pay an extra $20 a month per line.\nThere\u2019s no speed cap on browsing or downloads; music streams at 550 Kpbs, while games are played at 2 Mbps.\nVerizon and T-Mobile also launched new unlimited data plans recently. They\u2019re not perfect either, but worth checking out.