Is that a joke? My phone currently has 3 different phone numbers. And even if you did prevent VOIP's.
Consider this .... we all have relatives that have cell phone's who don't play League. Currently, in my household ... we have a 3 cell phone to 1 league player ratio. So now I can put 3 accounts into Clash if I wanted to.
If you want to prevent Smurfing .... use a IP / MAC address combo.
If IP is the same for multiple accounts .... Then is Mac Address also the same? Sure, there are some problems with this method also ... but they are far fewer than your "SMS" system.