Every day I see that more and more areas are moving to 10-digit phone number dialing. Calls made without dialing the area code first do not go through. This is because of the coming of Local Number Portability -- the ability for people to take their phone numbers with them when they change service providers.
My advice: Take some time now to program all your cell phones and speed dials with all 10-digits of the phone number you want to call. It doesn't cost extra to call with the full area code, exchange, and number and you don't need to put a 1 in front of it. "Yet," that is, though I'm sure that will be here in a couple years.
For now, it will primarily affect calls to cell phones, whether from a landline or from another mobile phone. Soon it will affect calls to landlines, as well.
BTW - Did I mention that I don't teach much anymore but now work for a major wireless phone service provider? Peace & Prayers