OK, I'm going to do you a solid then and tell you how to get round typing the PIN in every time you want to access your mobile voicemail. Go into your settings and find the voicemail number (on Android it's Menu > Settings > My Device > Call > Voicemail Settings > Voicemail Number, I don't know about iPhone), then at the end of the number add Pause (under the Symbol key on Android) followed by your PIN. Save and exit. Now when you dial voicemail, it will automatically enter the PIN for you.
You can thank me in beer.