With clash on the way, it makes sense.
It would be a fairly simple system.
Before you are allowed to play ranked, you register your phone number with Riot. After confirming that it is indeed your number, you are allowed to play ranked (or Clash) on the account you registered your number with.
You may not register multiple accounts under the same number.
You will be periodically required to re-confirm your number. If you change regions, you will be required to do so immediately.
This makes smurfs in ranked and account buying a non-issue.