For those who live other then the US or unlocked and have a non-functioning Voicemail button, here’s how to change the default number.
Step 1: Find your carrier�s voicemail number is (for example, in Singapore the SingTel voicemail number is 1389 and in Australia, the Vodafone voicemail number is 121)
Step 2: Click the phone button in your iPhone. (Green one, bottom left)
Step 3: Click the keypad tab.
Step 4: Type the following: *5005*86*yourvoicemailphonenumber# (just google for your phone carrier’s voicemail number).
Step 5: Hit the call button and wait a few seconds.
Step 6: Hit the voicemail button, and it will now call your voicemail.