Can you plug the phone that doesn't get an IP into the cable of a phone that is working. That will narrow it down to either a phone issue or a network issue. Also, check the network settings and make sure the phone is set up to get an address via DHCP and that it doesn't have a static IP configured on it.
Check the presence of the voice vlan all along the way(in case of multiple switches) from the phone port on the access switch to the router(DHCP Server). If it is ok, either enable CDP and issue the 'switchport voice vlan [voice-vlan-id]' on that port (applicable to cisco switches only), or if it is not a cisco switch, or you don't wish to enable cdp, set the 'Admin VLAN' network setting on the phone to the voice vlan ID. To set it, first unlock the phone with '**#' while in network settings menu.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...