WARNING: This does not work with 800 numbers using FireRTC!
Some people might not know how to do this but i think it is useful to be able to warn people that the number is a scam.
What you have to do is just simply change your number to the scammer's number in FireRTC or another program that allows you to change your number then call that same number. This should work to give you voicemail access but may not always work if the scammer has a passcode or someone else has put a passcode on the voicemail.
WARNING: If the number is not in service or the numbers posted no longer belong to scammers do not access the voicemails of those numbers!