Password Reset Server supports a third party called ProxStop to provide phone verification via phone call or SMS. To use ProxStop with the "Multifactor - Phone" or “Multifactor - SMS” questions, you will need to sign up for an account with ProxStop. ProxStop charges approximately $0.12 per call or SMS to a phone number in the United States. ProxStop also supports calls to other countries ( see their rates).
Sign up for a ProxStop account
Get your API Key from ProxStop (under their "API Services" section)
Add them to Password Reset Server (Administration | Configuration)
Add a "Multifactor - Phone" and/or “Multifactor - SMS” question to your Security Policy