How to change Voicemail number in iPhone?
Call : *5005*86*123456789# where 123456789 is the number you want to call when you press the Voicemail button.
For example, you can set your Voicemail number to your own iPhone telephone number. When pressing it, you will always get a busy tone. Thus, this will prevent accidentally calling Voicemail number.
Credits : http://janandjanuary.multiply.com/journal/item/68/68