numbers-shortest.py

print sum([1 for number in open('tel.txt').readlines() if number[0:3] == '511'])

# 100 loops, best of 3: 1.65 ms per loop

numbers-with.py

from __future__ import with_statement # if py2.5

with open('tel.txt') as f:
    print sum([1 for number in f if number[0:3] == '511'])

# 100 loops, best of 3: 2.07 ms per loop

numbers.py

f = open('tel.txt')
c = f.readlines()
f.close()

print sum([1 for number in c if number[0:3] == '511'])

# 100 loops, best of 3: 1.68 ms per loop