I don't know why it happens but I often have to do it with my iP6+ depending on what part of the city I'm in. If you find out any info regarding this...please post as I've be interested in knowing why I have to do this as well. And, as to note, it's not like I'm traveling far...I'm in a major city and the difference can be 10 miles on one direction or another an it'll play a part in if I need to dial the area code or not. Strange...
Honestly you should always dial the full ten digits from a cellphone no matter if the area code dialing system requires it. I have saved all ten digits on my cellphone contact list since 1997 in a state where it wasn't required for one of the area codes in that state until 2012 when the first overlay plan occurred.
It doesn't cost you any additional money to do so.
There was a overlay added nearby that might be affecting your dialing. http://m.sfgate.com/news/article/End-of-line-for-415-2nd-area-code-coming-for-5079322.php
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.