Yes, you can. However, I'm going to make a statement to let you know what this will do.
1. Anyone who picks up your phone (who knows how to access your phone) and presses the voicemail button will be able to access your voicemail.
2. Anyone who picks up your phone and checks the call log and sees your voicemail calls will see your voicemail pass code.
3. But it will allow you to check your voicemail without pressing your pass code each time.
Just want to give you the facts before you change these
If you still decide that you want to pursue this,
1. press the menu button at the home screen
2. select Settings
3. select Call settings
4. select Voicemail
5. (you should see *86) press the comma button two times and follow that with your pass code
6. press OK
7. press OK when it confirms the change
Now you'll be able to call your voicemail without entering your pass code.