Q: how can enable voicemail in ios 7.1 and setup greeting?

You may need to contact your carrier or provide more details of exactly how you are trying to set up the greeting and what happens when you try.

Following are instructions that may help: http://support.apple.com/kb/TI14

Also the "Contact your carrier" section (step 8) of the following leads to a webpage that gives details of which voicemail features your carrier supports http://support.apple.com/kb/TS4430