How many possible 10-digit phone numbers with the area code 617 are there? (any decimal digit can be the digit of a phone number. for instance the number 617−000− is a possible phone number)

If all digits are possible and we are restricted to the 617 area code, then there are:

10^7 possible numbers  

Because each digit can have 10 possible values from 0 through 9, you get

10*10*10*10*10*10*10 which is just 10^7

10,000,000  (ten million)