Authy's primary carriers for SMS and voice call verification / app registration are "best effort", which means we cannot guarantee reach to every phone worldwide. If you were able to receive SMS in the past but now cannot, your best solution is to see if you can temporarily use an alternate number (perhaps owned by a family member or friend) and install the Authy app. Once you have the app installed and can access your tokens via the app, you may turn off multi-device to prevent access from other members of your family! And update the phone number in the app later without delay.
We do not fully support landlines and recommend users avoid them for use with Authy. Although we support phone calls to verify your identity and install an Authy Desktop app to your computer, we've found that if the phone call doesn't work you will be locked out and there are no other means to authenticate. Please don't use Authy with a landline.
We may have difficulty reaching VoIP texting services, as some do not accept SMS from US short codes. Additionally, VoIP numbers are often very easy targets for hackers compared to phishing attacks on a phone company account - all an attacker needs to access your VoIP is usually just a password. If your VoIP phone number is only locked with a password, this defeats the purpose of two factor authentication, and you are back to just a single factor! Use at your own risk and if necessary, change the number on your account to a mobile carrier.