Duo’s telephony usage includes activation and authentication SMS messages and authentication phone calls for both end users and Duo administrators. Duo debits your telephony credit balance for each authentication call or SMS message sent (as per the rate card).
If you ever exhaust your telephony credit balance, your users will not be able to receive authentication phone calls or text messages. They will still be able to authenticate using non-telephony methods: Duo Push, Duo Mobile passcodes, and hardware tokens. Duo Push and Duo Mobile-generated passcodes — are app-based and have no telephony cost, but require a smartphone or other mobile device.
Paid accounts are issued credits yearly, and may purchase extra telephony credits as needed. Duo Free account holders can also purchase additional telephony credits.
NOTE: When using the DUO mobile app – Telephony credits are not needed.
Link to addition Telephony Credit information here: https://duo.com/docs/telephony-credits