Sorry for being cynical, but judging from all these services coming and going all over the place, any of the following is going to happen within a year: 1) authy gets bought by $COMPANY. With some likelihood it's not a company I want to have my phone number. 2) authy runs out of money and needs a revenue stream. Thus it changes the TOS and starts selling phone numbers. 3) authy shuts down. Who knows what's going to happen with my phone number then.
No. I'd rather use any other HOTP app that doesn't require any personal data (which is one of the basic ideas behind HOTP).
Authentication and my phone number are the two things you have to really prove you have got your act together before I trust you with them.