Q: I have been trying to set a new voicemail greeting on my iphone 4s but every time i check the 'save' button it says Voicemail Unavailable, try again later. Any help would be greatly appreciated! thanks

Voicemail is a Carrier feature... Contact your Carrier...

But first... Try This...

Close All Open Apps...  Perform a Reset... Try again...

Reset  ( No Data will be Lost )

Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.

http://support.apple.com/kb/ht1430

Voicemail is a carrier responsibility. You need to make sure you have cellular signal when trying to do this. Try the information in this support document http://support.apple.com/kb/TS4430