You can try editing it from settings >swipe left to applications > phone > default voice number
But I believe that does not work. Best way to do it is by adding your voicemail number with the pin as a contact.
Check this thread out
Great question! It's important to make sure that your device has the features you need most. You can add a "pause" to the Voicemail contact, so when you call the voicemail it will pause then put in the password. Click here for the instructions http://bit.ly/VxCF6M . Have a great day!
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