How to prefix an area code when dialing with iPhone?
It may not be a perfect solution, but what I do is store all numbers in my contacts with 1 and the area code. That way it dials appropriately no matter where I am (it doesn't change the billing details - a local call is still a local call, whether it includes the area code or not).

Also, maybe this is a carrier specific thing, but any texts I receive from numbers not in my contacts show up with the full +1(123)456-7890 formatting.