General Connectivity: Have you tried the following?
Is the handset plugged in the right spot? (handset = the part you talk into)
Have you unplugged and re-snapped the cable in at both the phone & wall?
Is the network cable plugged in the LAN port on the phone?
Are the ends of the network cable in good shape (have working clips)?
Have you replaced the network cable?
Have you moved the phone to a different wall jack?
Did moving the phone to a different wall jack work?
Is anything like a computer plugged in to the phone?
Have you power-cycled/rebooted the phone?
What is the IP address showing on the phone?

Advanced Networking: Have you tried the following?
Is there a firewall/router/switch in the office or one specifically for this user in the equipment room?
Is the phone plugged directly into the wall or into a switch or other device?
Have you tested the wall jack with a laptop to see if you can get on the internet?