From the Cisco website:
CISCO 7900 SERIES IP PHONES
Best Practices for Cleaning and Troubleshooting Cisco IP Phone Hookswitch
Introduction
We have completed a thorough analysis of a large number of Cisco IP Phones that have been reported as hardware failures and our findings reveal that the great majority of these phones did not have any determinable failure. The standard by which the phone industry measures phone failures is based on Bell Corp/Telcordia standards and the standard acceptable failure rate is set at 4%. The Cisco IP Phone's overall failure rate is well under the acceptable industry average. Many of the problems that have been reported as hardware failures are really either operating or cleaning issues. This document describes some common troubleshooting steps that should be taken before attempting to replace the Cisco IP Phone.
Cleaning and Troubleshooting Cisco IP Phone Hookswitch
The following section highlights some guidelines for phone usage and what to look for if you suspect a hookswitch failure.
Cleaning
Only the use of a slightly dampened soft cloth should be used to clean or wipe down the phone. Do not use any liquids or powders directly on the phone for cleaning or other purposes. As with all but weatherproof electronics, liquids and powders can contaminate the components and cause failures.
Signaling
Check to see whether signaling is working properly between the phone and the Cisco CallManager. Use the speaker button to answer the phone and/or take the phone off-hook. If the call is answered and/or dial tone is received, signaling is working properly.
Cradle Clip
The phones are designed with a reversible clip in the handset cradle area. The clip is used with the tab out when the phone is wall-mounted. Check to see whether the cradle handset clip is in the wall-mounted position (with plastic tab protruding upwards). If the phone is on a desktop, slide the clip upward to remove it, rotate 180 degrees, and slide back in so that the tab is hidden.
This tab can interfere with the handset as it is replaced on the base (in the cradle), causing the hookswitch to remain in the up position. When the handset is lifted later in an attempt to initiate a new call or answer an incoming one, the hookswitch is not activated. If answering a call, the phone will continue to ring; if placing a call, no dial tone will occur.
LAN Cord
Check to see whether the dark gray line cord / LAN cable that was packaged with the phone is being used. If a different patch of cable is being used, position it to pass out the side of the phone, between the base and the footstand.
Other cables such as Cat-5, Cat-5E, or Cat-6 have larger diameters and will not pass under the rear of the footstand without raising the phone forward. The footstand is designed to allow the most number of positions to eliminate glare from the display. When the phone is adjusted to the most vertical position, larger diameter cords actually force the phone forward to the point where the handset may not sit firmly on the hookswitch. This creates false off-hook conditions.
Keeping the phone one notch from the most vertical position is recommended where practical to ensure that the handset is firmly resting on the hookswitch.
Self Cleaning Hookswitch Contacts
The hookswitch contacts design uses a wiping action to self-clean the contacts. Periods of nonuse of the phone allow air impurities such as dust and other contaminants to hinder contact performance resulting in intermittent operation. Rapidly pressing and releasing the hookswitch a dozen times or so will clean the contacts.
If none of the solutions above remedy the situation, RMA the phone.
For more information the URL:
http://www.cisco.com/en/US/products/hw/phones/ps379/products_tech_note09186a00801c0810.shtml
