By blocked I don't mean banned, I meant "locked". After phone re-verification accounts are usable again, but if I try to do any following again, they get blocked again, asking for phone re-verification again .. and so on.
My accounts are not that old, 1-2 months, I have 10 accounts, using no proxies and was doing 500 follows with 500 unfollows daily, semi-automated with a chrome extension called "Twitter follower" that adds some small delay between each follow. For unfollowing I was using "unfollowerstats", a free web app similar to crowdfireapp, which also adds a bit of delay between each unfollow. Maybe a bit reckless for not using proxies, I know, but it was working fine up until now.
When I tried to follow manually same thing happened. Tried to space out the followings: followed one person, waited 1 minute, followed another, account got blocked again after only 2-3 followings.
It is possible that account age is a factor, since my accounts are fairly new, it gets triggered after maybe 100 follows. The people reporting with 700+ daily followings could be using older accounts.
Another thing could be my IP address getting flagged somehow, so now any suspicious behavior from my IP triggers sms re-verification. Though I beleive account age is more likely, since most of the people here reporting it have fairly new accounts.