I use Google Voice to handle my voicemails and authenticate with my google login. Another option may be visual voicemail through your carrier, for me on VZW they didn't require me to enter my pin.
Lastly, you could set up a contact to dial you VM number and enter your pin automatically. You just need to add the correct number of commas (,) to pause between the number being dialed and the pin being entered.
Example: 555-555-5555,,,,1234