I always ask people beforehand if it's a local call, because historically, local calls are no problem, but long distance calls are limited.
This isn't really a reference question, but I get asked this all the time and I'm honestly curious about it - here's a typical exchange (keep in mind I work at the library in Chelmsford, MA, which is in area code 978):
Patron: Can I use the phone?
Me: Sure, is it a local call?
Patron: Uh, I just need to call my mom.
Me: Okay, what's the number? [I always dial for them, to make sure they get an outside line and don't dial 911 accidentally, which does happen with our phone system]
Patron: It's 603-423...
As I get older (and as kids get younger), I've been noticing that fewer and fewer kids have any idea what you're talking about when you say "local call."
When they say "603" (New Hampshire) or "617" (Boston), I will sometimes say something to the effect of, "hey, a different area code is not a local call," and the response from kids is invariably, "we live in Chelmsford, it's my mom's cell phone."
I don't have a cell phone, so I don't know if there is such a thing as local and long distance calls on them, or if everything is charged the same (or just depends on time of day). But wow, the whole local/long distance thing was a big part of my childhood, so it's kind of stunning to think of kids growing up with no concept of that. Depending on how often people move around, a kid's friends could all have cell phones with different area code numbers, and have no idea why*.
But then again, I guess people don't really dial numbers any more anyway - it's just scrolling through the contacts list and clicking a name. Which means the reasoning behind area codes is destined to become historical trivia like the interstate numbering system, or an anachronistic relic like the phrase, "don't touch that dial."
*Tangentially, an old rant of mine is how the FCC dropped the ball when they started issuing phone numbers for cell phones. Instead of issuing cell phone numbers with area code where the phone was registered, and thus running out of numbers and having to slice up area codes and develop new codes (giving rise to situations where the "area" codes make no geographic logic, like 440 in Ohio), they should have created new area codes just for mobile phones. Which would have also helped out with making sure cell phones were always on the Do Not Call List, as they could just forbid those area codes from being called.