Troubleshooting Cisco IP Phone Registration Issues

How to troubleshoot Cisco Phone Registration issues with a UC500 or Cisco Unified Communications Manager Express
There are a variety of reasons an Cisco IP phone using SCCP may not register with the Call Control Agent.
Your phone is not registering? Lets look a good trouble shooting methodology for Cisco IP Phones.
To Start WORK THE first three layers of the OSI MODEL FROM THE GROUND UP.

• Physical Layer.  Does the phone power on?  Does it pass the Power on Self Test? (during POST the lights cycle through the buttons on the phone) Are you plugged into the appropriate switch port on the back of the IP Phone?
• Data Link Layer Does the ip Phone recieve the appropriate Voice Vlan from the switch or UC500 switch port? Depress the settings button and under network configuration you should be able to find this information. 
• Network  Does the phone get a DHCP assigned IP address?  Including option 150 the TFTP server?  If it does, can the IP be pinged from the UC or router?

Next, make sure the 4 major  commands to RUN CME are present at the command line.

• max-ephone This command is found under telephony service configuration.  It specifies how many phones the system is provisioned to support.   The default is 0 so if you don't have the command you will never get the phones to work.
• max-dn This command also is found under telephony service configuration.  It specifies how many directory numbers the system is provisioned to support.   The default is 0 so if you don't have the command you will never get the phones to work as well.
•  ip source-address 10.1.1.1 port 2000
• ip source-address another telephony service command.  This specifies what ip address CME will speak the SCCP call control protocol from. The default port is 2000 and does not need to be entered. I like to use a Loopback address in my installs since a loopback is always administratively up.
• create cnf-files Also found under telephony services configuration.  This command make the router create configuration files for each phone in the system that stay resident in memory.  If the phone can't download it's configuration file, it will never register.

The above will fix 98% of phone registration issues.

• Other things to check are for appropriate firmware loads for the phones on the router/Uc500 
  tftp-server statements for firmware and load statement for firmware associate to phone models, but most times a phone with out of date firmware will usually register.