It's hard to tell from context whether you're not from the US or you're an American who has never used seven-digit dialing.
In the US, the first three digits of a ten-digit telephone number are the area code. Traditionally if you were calling someone who was in the same area code you were in you would not dial the area code but only the last seven digits of the telephone number. If you needed to call someone outside your area code, pressing 1 first allowed the telephone network to know that you were dialing a ten-digit number.
It used to be that when an area code ran out of telephone numbers, it would be split into multiple area codes. This meant that some numbers would change in the first three digits, which is inconvenient. The modern practice is to add a new area code that applies to the same geographical area as the old one, but that means that someone living next door to you may have a different area code than you do; the solution is to make everyone dial the full ten digits regardless of area code.
I think landline practice (7- or 10-digit dialing) still varies by region, but all modern cell phones use ten-digit dialing.