Probably because you have a password protected voicemail with your provider! If the 4S is spontaiously asking at random times, than it could be a software issue! What you want to do is sync your contacts to the computer as well as store any other information you can to your computer such as your music, apps, and pictures to iTunes. (Photo's isn't actually saved in iTunes but to the computer, you can just see the file through iTunes)
Assuming your on a windows computer, here are some helpful links!!
To sync photo's to computer:
And if "autoplay" isn't enabled, here is how fro windows:
And here is how you can transfer your purchased content including music, video's, and apps to iTunes:
And this is how to sync contacts to iTunes (it actually goes to outlook)
And as a pre-caution, this is how to Back up, update, and restore!
Update and restoring: http://support.apple.com/kb/HT1414
Hopefully I provided some useful and helpful information!
Hopefully I was some help