Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5F136 Safari/525.20)
I have had a similar occurence.
For example, if I put in a number under my contacts that says 645-1643, and the number that comes up as 603-645-1643, my iPhone won't notice that number because I don't have it set up with the 603. Same goes for the 1 in front if the area code. If I don't have it under my contacts with a 1 before the area code, my phone will not display the incoming caller's name.