For the purposes of entropy, you need only consider 10 valid choices for each symbol of a phone number so it's closer to 33.21 bits (10 * (log(10) / log(2))) and smaller still when discarding impossible area, trunk & subscriber numbers.
So given than 80 bits is much bigger than 30-40 bits, if I know someone's hash I can very easily narrow down their phone number to one or sometimes two candidates.
