Because you are using a non-Cisco switch that provides 802.3af PoE. The problem is that the 7940,7960 and other older Cisco phones use a pre-standard power and not 802.3af PoE which this manufacturer uses. The 7911 should be good to go :)
Q. What IP phone models support the Cisco pre-standard method of powered device detection?
A. These Cisco IP phones can accept Cisco pre-standard PoE from a card integrated with a Cisco Catalyst switch or a Catalyst in-line power patch panel:
7910G + SW
These phones can draw local power from a power cube (CP-PWR-CUBE-2=) in addition to a country or regionally specific power cord (CP-PWR-CORD-xx=).
Q. What IP phone models support the IEEE standard method of powered device detection?
A. These Cisco IP phones support IEEE 802.3af PoE:
Note: These Cisco IP phones support both the Cisco pre-standard PoE and IEEE 802.3af PoE:
SIP traces provide key information in troubleshooting SIP Trunks, SIP
endpoints and other SIP related issues. Even though these traces are in
clear text, these texts can be gibberish unless you understand fully
what they mean. This document attempts to br...
Please find the attached HTML document, download and open it on your PC.
This provides an easy to use form where you simply answer a few
questions and it will render the proper jabber-config.xml file for you
to copy/paste. There is built in logic to verif...
CUCM Database Replication is an area in which Cisco customers and
partners have asked for more in-depth training in being able to properly
assess a replication problem and potentially resolve an issue without
involving TAC. This document discusses the bas...