Welcome to Android Central! It would have to be an app you can install from the Play Store or some other source. Trying to install the Bell firmware on a Verizon phone would likely cause a lot of bugs, or possibly brick the phone.
Are you saying that you want to change the voicemail number that gets dialed when you long-press the "1" button on the dialer pad? Open the Phone app, tap More>Settings>Call Settings>Voicemail, and look for the options there to change the voicemail number.