[jfornear]

I recently learned this the hard way. I changed my phone number after moving cities, and AT&T just leased my old number to someone else!

[Tijdreiziger]

There is a finite supply of numbers. Not recycling old numbers and just throwing them in a hole forever wouldn't be very prudent.

[toomanybeersies]

There is an infinite supply of numbers. You can just add extra digits. Mobile phone numbers used to be 8 digits in NZ, then became 9 digits when they needed more numbers.

[cbluth]

Numbers aren't limited to the amount you can count, there will always be more.

[manigandham]

What did you expect was going to happen?

[Fnoord]

I'd expect it to be reused _eventually_; not right away though.