For the code 21 diversion the optus site says: "Add diversion: ** (code from above) * (target phone number) ** (enter ring time: 05, 10, 15, 20, 25, 30) # (press call)"
Is it necessary to add the ring time?
Mobile phone plans, SIM card activation and porting issue
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If the 21 code represents diversion unconventionally there is no need to set the ring time. You can refer to the below link for more information.
https://exewiki.exetel.com.au/index.php ... bile_Optus