Let's start by getting this out of the way - cheater's suck. I have no sympathy for people who cheat and they get what they deserve. Valve thinks they need to be banned - not just based on user account or email, but phone number as well. At this point you don't HAVE to provide a phone number, but if you have - you won't be able to use it again - and you'll have to create a new Steam account with a new email address and say goodbye to your old library. That's rough - and I applaud their crackdown.
Kotaku notes that even though carriers tend to re-issue phone numbers, the chances of getting a nefarious Counter-Strike player's digits with a new phone and being wrongly banned yourself are pretty low. Beyond that, CS:GO's matchmaking forbids using a VOIP number, which rules out that loophole.