iPhone 4 on iOS 5 - can't change voicemail access number away from carrier
 

My wife's Android phone was easily changed to use Hullomail voice access number instead of T-mobile's expensive voicemail.

My new Orange contract also charges for voicemail so I've setup Hullomail. The app works, but I wanted to fix the voicemail button too as a shortcut when out of a data area to be able to access voicemail, but the *# or ** 5005*86*[voicemail number] settings change it to what I want when queried with **5005*86# but the voicemail shortcut button on the iPhone seems to be locked despite turning off/on, resetting etc.

Loads of people on various forums moaning about this, wondered if anyone on here new how to fix without jailbreaking?

Pretty sure you can't without Jailbreaking. The function is allowed and disallowed in the carrier profile (carrier.plist), all the UK networks block you from changing the button.