As I said, not defending them. They are likely doing dozens of other things as well. But using phone numbers is a quite effective method of hindering spam/bot account creation - in most countries in Europe at least getting a prepaid SIM requires ID nowadays. Not that Twitter would go as far as to inquire ownership records of phone numbers... but/so you could still go and buy 100 SIM cards if you wanted to, but it'd be way more expensive than just spawning new email addresses.