Interesting, but it doesn't seem to work that way on my cell phones. On my RAZR, when I go to enter a URL, the phone defaults to text, so the number = "certain letters" system does not work. So, (for example) I might want 477 to mean "IRS," but it would type "GPP." I can switch to numbers, but that takes an extra keystroke...
On my BlackBerry Pearl, there are two letters only per key - and not all of the letter keys have a number equivalent. That phone, too, also defaults to text when I select to "go to" a URL.
A lot of the new phones are trying to make typing easy(ier) on a small keypad, the the number-letter equivalence on many newer phones does not match up like on most of the current phones.
Then there is the use of PDA with larger keypads with a qwerty layout--no match at all...
BTW, my RAZR is with Cingular and the BB/P is with T-Mobile. I'll check with my daughter to see how it works on her Verizon Chocolate. Meanwhile I'll be keeping away from those NNN and NNNN combinations. (But I do have a couple of good LLL .mobi domains.
