I'm not totally sure, but I think it's helpful if the call comes from a local area code. But, many of us have legacy area codes from places we lived 10-15 years ago, so it may not matter too much.
In all seriousness, though, calling from a local area code does make a huge difference.
If you don't have a local phone number for some reason (recently moved, etc.), then you can still call and identify yourself by name and say that you're a constituent (note: say "constituent", not "voter").
