Expanding, US phone numbers are xxx-yyy-zzzz, where x is area code, y is prefix, and z is number.
It used to be that you did not have to dial the area code if you were calling a number in your same area code. So if your phone number was 415-591-0726 and you were calling 415-327-0914, you could omit the "415" part.
The prefixes ?11 were reserved: 911 for emergency, 411 for directory information, 611 for telephone company support, 711 for TDD / relay for the deaf, then 511 for road conditions I think, and then 811 for "call before you dig", aka please don't backhoe our fiber optics.
Many cities now also have a 311 service as a kind of non-emergency municipal call center for things like paying parking tickets or reporting potholes. My favorite use of this was in the early 2000s when it was used to trace the source of the mysterious "maple syrup events" in New York. People were encouraged to call 311 the moment they smelled the maple syrup odor and give the operator their location. By plotting the times and locations, they were able to trace it to a factory in New Jersey that was making artificial maple syrup flavoring from fenugreek seeds: https://web.archive.org/web/20190208102307/http://gothamist....
It used to be that you did not have to dial the area code if you were calling a number in your same area code. So if your phone number was 415-591-0726 and you were calling 415-327-0914, you could omit the "415" part.
The prefixes ?11 were reserved: 911 for emergency, 411 for directory information, 611 for telephone company support, 711 for TDD / relay for the deaf, then 511 for road conditions I think, and then 811 for "call before you dig", aka please don't backhoe our fiber optics.