Voicemail setup

Im having issues setting up my voicemail on my iphone 4 [version 4.1] with t-mobile [UK]
I have dialed 222 to setup it up, then called the voicemail number, created my PIN and the voicemail message, thats all fine. But when I dial the mobile number [from another phone] it does not kick into the voicemail, but rings out and states the person is not there after about 20 rings.

If I setup Call Forwarding to the voicemail number through Setting - Call Forwarding, then dial the number it goes straight to the voicemail message fine.

But cant work out how to setup the voicemail normally, ie goes to voicemail message after 5 rings etc. When I dialed the voicemail number and went through the option to create PIN and message, there was nothing about duration of rings before voicemail starts and can not find anything online to set it up.

Any help would be grateful.

I have not spoken to T-mobile about this, presumed I can setup/configure the voicemail service myself either through my phone settings or on the voicemail numbers options. Is that not correct?

The voicemail service itself works OK, because if I just setup the Call Forwarding, goes to voicemail OK.

Phone works fine other than this voicemail problem.

Thanks for the replies. But surely there is just a default way to setup voicemail for iphones? Doesn't matter which cell provider Im with, is there not a default way to setup voicemail for iphones?
I maybe just missing something obvious or is it not enabled/configured correctly for my account?

Just want a normal call to my number to go to voicemail after X amount of rings.