all calls to voicemail ring at 20 seconds, which is approxc 4 rings, this is hardcoded in the GSM HLR and can not be changed via a code or calling into a rep and complaining to them.
if you want to disable voicemail, you can if its a standalone option just ask to remove the standalone feature from your account, your monthly fees will decrease by doing this. if you have a value pack and voicemail is part of it, its not so easy, you would have to inquire to a CSR and they can check if a suitible value pack exists without the voicemail, or they would have to charge you for each of the services individually minus the voicemail soc code, does that make sense?
Voicemail is a feature I do not use anymore, in fact, I have never even initialized it. I have emailed Rogers and inquiered about getting a plan without it. I find it takes me a lot longer than 4 rings to get to my phone most of the time!! I have heard other peoples phones ring more than 4 times though, so I know that it is posible!! I will keep you posted!!