Duo provides a wide range of options when it comes to authenticating a user login. We expect that most Duo users will prefer to use either Duo Mobile or Duo Push, neither of which consume any telephony credits and can be used without limit.Duo also supports two-factor login via other methods, such as automated voice calls or SMS messages. Duo incurs a small marginal cost when an automated voice call is made (long distance telephone charges) or an SMS message needs to be sent (SMS charges), with the exact amount depending on the source and destination of the call or message. Telephony credits are the way that Duo ensures those marginal costs don't get out of control. Telephony credits are purchased in advance as a non-expiring credit in a pool that covers all user accounts at a site. In the United States, voice calls for login confirmation incur a two (2) credit charge; SMS messages for login confirmation cost one (1) credit in the US. Rate cards for other countries are available. Telephony credits are purchased directly from Duo after you have enrolled in the program.