validate phone number

phone.py

#!/usr/bin/env python

def isPhoneNumber(text):
    if len(text) != 11:
        return False

    for i in range(0,3):
        if not text[i].isdigit():
            return False

    if text[3] != '-':
        return False

    for i in range(4,7):
        if not text[i].isdigit():
            return False

    if text[7] != '-':
        return False

    for i in range(8,11):
        if not text[i].isdigit():
            return False

    return True

print('666 666 666 is a phone number: ')
print(isPhoneNumber('666-666-666'))
print('Moshi moshi is a phone number')
print(isPhoneNumber('Moshi moshi'))

Play around with the interactive shell to see how exactly the ranges expand:

% python                                                                                                                                                                                                !2070
Python 2.7.10 (default, Aug 17 2018, 17:41:52) 
[GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.0.42)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> range(0,3)
[0, 1, 2]
>>> range(4,7)
[4, 5, 6]
>>> range(8,11)
[8, 9, 10]