4S Set Number of Rings Before Forwarding to Voicemail?

hey,

i'm trying set number of rings (or time in seconds phone rings) before forwarding voicemail.

i've looked few guides , determined, *#61# voicemail number +16479971130.

then, use code *61*+16479971130*11*35# attempt change ring time 35 seconds.

error in response.

"error performing request
unknown error

setting registration failed
voice call forwarding when unanswered"

called carrier, fido (toronto, canada) , weren't helpful. told me consult iphone manual.

how should proceed?

 

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