Lost password to voice mail on 4s. Help
IPhone 4s locked. Lost password. How do I recover recover or replace password?
iPhone 4S, iOS 9.1
The title to your post states the voicemail password is lost but the post itself says the iPhone is locked. That is two different things. See the following for each:
iPhone locked: see If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
voicemail password: contact your cellular carrier customer support