I'm only guessing but the two sentences "They need THE cell phones now" and "They need cell phones now" have different meanings. In the first case, it sounds like they already own cell phones and they need them. In the second case it sounds like they need to go buy cell phones. That's my thought process for what it's worth.
I believe, as long as you have no typos (including missing accents), you can writeeverythinginoneword or int roduce arbi trary sp aces (though I'm not sure if more than one per word will work - but I expect it would) as far as Duolingo's checking algorithm is concerned. An error would have a different cause (like a missing valid translation).
I left out "the" and Duo said I needed the article "the." Maybe translating from English to Spanish may require certain articles. However, translating from Spanish to English does NOT require articles. Since we don't know if cell phones need to be purchased or if they already have cell phones then it's difficult for many to answer correctly.
I had this problem so often when I first started using this program that I skipped any listening exercises for the first several months. It really impeded my learning. Then I switched from using Explorer to Chrome, and I haven't had a problem since.
I highly recommend using a different browser. Duo seems a bit preferential of some over others.
Sam: I disagree. I have over 27 years experience in wireless telecommunications wit ALL the major US carriers (ATT Wireless, Verizon Wireless, Sprint and T Mobile and the term "cell telephone" is a common term as well. You can not be a native speaker or familiar with the industry.
From the English course description pages (es, fr & de - i didn't bother to look beyond that):
On Duolingo, you'll learn American English, but you'll be understood in any English-speaking country.
Also, it only takes a cursory glance at spelling / word choices to see that American English is what's being used by Duo, though non-american spellings or words will generally be accepted. Report the sentence if it doesn't.