Hi
@silasp, you could try the following MMI code. It used to work in most carriers, as did the facility to alter the number of rings in 'voicemail'-'call forwarding'-'unanswered' in 'settings' but for some reason most carriers have blocked the facilities.
I'm with three carriers only one of which allows me to change the number of rings using either method, so I guess it's bit of a long shot, but worth a try.
"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*+447781233123*11*30#)
Which is the generic voicemail number for UK Carrier '3'.
Then key in *#61# to confirm the new setting"
I have just doubled checked my two other carriers and one of them, 'ee', does still allow changing the number of rings by either method. My other carrier is BT which does not allow any changes by either method, although they may respond to a request if I asked them, but I'm happy with it as it is. To be honest I don't know what the delay is set to, if I hear the ring I answer it, if I don't answer it's because I haven't got the phone with me so it goes to voicemail and the number of rings is of no consequence.