When the callback type you receive is either VerificationCallback.TYPE_MISSED_CALL_RECEIVED or VerificationCallback.TYPE_OTP_RECEIVED. This implies that a drop call / SMS has been received on the user's mobile number on that smartphone. Please note that for getting the TYPE_OTP_RECEIVED callback, your app needs to have the Google SMS retriever hash code configured on Truecaller's developer portal ( while creating your partner key ) so that the SDK can auto read the incoming SMS and share the OTP with you in this particular callback method.