As it currently stands, you have entrusted your precious numbers to them. Without 2-factor authentication, you stand a very real risk of someone being able to hack your account and seize control over these numbers. These, in my eyes, is more catastrophic than the loss of an e-mail address.
Kindly vote for the suggestion I have posed up at Ring.to's feature request forum - Support Two-Factor Authentication! (https://bandwidth.uservoice.com/forums/172463-feature-requests-improvements/suggestions/6218088-implement-two-factor-authentication)
As I see it, its a win-win situation: we get better protection, and more folks will consider porting their precious numbers to Ring.to. I think its a great service and a very handy place to park numbers and I'd hate to see the service die as reports of hacked accounts and number theft surface.
Thanks so much in advance!