Need to change the voicemail number set to my Instinct

I am a Bell mobilitity subscriber and there phone support is horrible.  I just received my Instinct today and I need to change my voicemail number that is set in the phone.  Where I am located, I have to use #902-555-1234 (where 555-1234 would be my phone number).  The phone currently has set into it 1-902-555-1234,
Can someone please tell me how I would go about changing this?  In the settings I have the option to add 'other' but not to modify the original number....