Samsung Omnia 7 Voicemail Number Edit - UK Tmobile How?

[whoosh]

Hi - newbie here. We have a Samsung Omnia 7 on a contract from TMobile UK and as you know TMobile charge 10p + VAT for calling your voicemail. SWMBO keeps accidentally pressing the voicemail icon on the bottom left of the phone dialling screen and keeps getting charged 10p. We have asked TMobile to turn off voicemail - so people cannot leave a message but - the accidemtal dial from her continues. I can get to the screen toedit the voicemail number and I can enter a new number in the field (150) but when I save - the numberdoes not change.

Any fixes?

Does the phone need to be unlocked to do this?

Cheers