Hi,
You could try the following two ways to check and change your settings, both work on my HTC M9, M7 and Desire S (all on different carriers) :-
1. Open your dialer and go to Settings, tap Call forwarding. You should get a network response giving you the voicemail number and status for the three forwarding events ie 'unreachable', 'busy' and 'unanswered'. Tap on 'unanswered' and it will show you the current delay with the option to change it.
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2. Open your dialer, key in *61*[voicemail number]*11*xx#, where xx is your required delay in seconds - 05,10,15,20,25 or 30, and hit Call/Send.
(in my case it would be :-
*61*+447953222222*11*30#)
Then key in *#61# to confirm the new setting.
Hope this helps.