how to change login verification to email as default

No option to default to 'email only' unfortunately - an SMS is always sent with a code when you request to login so you'll need to request it to be sent via email each time. Most networks utilise this to help with account security. Also with recent changes made to Smarty's authentication - if you trust the browser/device that you use, you'll remain logged in for a longer period of time (and therefore be asked to login less often).

Good to know: if you chose to have the text sent via email, the code that was sent via SMS will be invalidated. 

SMS only i'm afraid.