When sending Voice and TextToSpeech messages, these are the potential error codes:

Status Result Explanation Maximum Attempts
Success SentOk Voice call received and answered successfully  
Success Max Duration Voice call received and answered successfully and the Max Duration limit (LIM and/or LIMB) was reached  
Success AnswerPhone Voice call received and answered successfully by an answering machine/voicemail  
Success AnswerPhone Dropped Voice call received and answered successfully by an answering machine/voicemail. The call was dropped due to your call's settings.  
Failed No Valid Option Selected 'Require Keypad Button Press' (KeypadOptionRequired) has been enabled for this message and the recipient didn't press a valid keypad button  
Failed Busy Recipient responded with Busy tone  
Failed No Answer Call unanswered within 60 seconds  
Failed Fax Machine Voice call received and answered by a fax machine (not a human)  
Failed Modem Voice call received and answered by a modem (not a human)  
Failed Invalid Voice Number (RP-Max) Mobile number is invalid (too many digits for the supplied network/carrier) 0
Failed Invalid Voice Number (RP-Min) Mobile number is invalid (not enough digits for the supplied network/carrier) 0
Failed Bad Number (NRD) Number is invalid (not owned by any network or carrier)  
Failed Bad Number (OC) Number is invalid (recipient's network or carrier rejected the call)  
Failed Bad Number (Unallocated) Number is invalid (recipient's network or carrier advises the number is unallocated/unused) 1
Failed Bad Number / Congestion Recipient's network or carrier is unresponsive (likely overloaded with no available capacity) 1
Failed Bad Number/No Answer (EM) Call completed with 0 second duration (likely an operator message stating the number is unavailable) 1
Failed Bad Number/No Answer (NUR) Number is invalid (the recipient's telephone equipment is disconnected)  
Failed Bad Number/No Answer (RTE) Recipient's network or carrier failed to connect the call  
Failed Temporary Failure (CCD) Recipient's network or carrier closed the connection when the number was dialled 1
Failed Temporary Failure (CR) Recipient's network or carrier advises the number is valid but is experiencing a temporary fault 1
Failed Temporary Failure (NCC) Recipient's network or carrier is unresponsive (likely overloaded with no available capacity) 1
Failed Temporary Failure (NOOO) Recipient's network or carrier is unresponsive (likely temporarily offline) 1
Failed Temporary Failure (NTF) Recipient's network or carrier advises the number is valid but is experiencing a temporary fault 1
Failed Unspecified Hangup (NU) Unknown failure reason (the Recipient's network or carrier is supplying an unknown error code)  
Failed Call Timed Out Recipient's network or carrier accepted the call but failed to route or reject the call (no result provided) 1
Failed Bad Audio Format Your submitted audio files are in an incompatible or corrupted format (should be WAV or MP3) 0
Failed Job Aborted Operator or User aborted the message 0
Failed No Permit Record Internal TNZ error; contact TNZ Support 0

 

Maximum Attempts means the system will limit attempts to the destination number.

For example, if you have set Retry Attempts=3 and the call fails due to Bad Number (Unallocated), the system will not automatically retry the call to that failed number (it will continue with other Contacts and Alt Phones as normal).