iPhone 4s-Voice mail Button-Settings required

Hi Apple and all

I have an iPhone 4s in London UK. it is unlocked so I didnt want to go with expensive contracts like O2. So I am using a cheap contract from company called TalkMobile.

Talkmobile provide better signal and cheaper line of sim only contracts. This has worked well for me as I am now paying £8 a month for my 300 min contract and 500MB data in comparison to £21 O2 contracts.

Talkmobile dont have Visual Mail service but have voice mail service that they charge for it. So I switch it off. (dialed 1210) that mean no one calles my voice mail. I suspriped to an app voice mail service called HulloMail. and that works great for me because I use it to personalise my voice mail and allot more security on it.

My question is.

How can I change the voice mail button to point to Hullomail app instead.

or

How can I disable the voice mail button all together.

or

How do I switch off the voice mail button with a password or something.

Reason why I want it changed is because even though I disabled people calling to my voice mail via the network provider. The iPhone will still call my voice mail if I press the voicmail button and I will be charged £0.10p for everytime I press it, to be honest at the moment its nusence to me, because MY FAT Finngers press it everytime I press the call app and accidantely (with my FAT Fingers) press it.

Surely this was a bad design by apple in the first place where by users cant disable, change, restrict or modifiy the voice mail button. If this cant be done then need to be fixed by Apple.