Symantec VIP manager provides dashboard overview of the credential validations. This KB article provide insight why you may see more unsuccessful credential validations than the successful ones.
VIP Manager
If a user in Symantec VIP has more OTP enabled credentials like SMS, Email and Voice, the validation of the OTP is done against all these credential unless a match is found, in this case if the OTP is originated from Voice credential then the VIP manager will capture two failed Cred validation and 1 successful( Voice Cred is third in the list) . This is the reason customer may observe more unsuccessful Credential validation than the success ones.
If user authentication is done via VIP, then the User Authentication graph should be looked at for the accuracy. User Authentications is also part of the VIP manager Dashboard