Since upgrading to iOS 8 I can't get my voicemails. It wants me to set it up as if it was new, so I put in the correct password and in the end it says my old password is incorrect. Seriously?
Call your carrier and reset your voicemail password.
They will send you a temp one and then once you setup visual voicemail you can go in settings/phone/change voicemail password and change it again to one your remember.