This sounds like a PoE problem. The 7945's are a Class 3 PoE
device and will not work on "older" switches that provide Cisco
Q. Do the Cisco Unified IP Phone 7965G and 7945G models support Cisco inline power?
A. No. The phones support only IEEE 802.3af PoE (Class 3).
Q. As Class 3 IEEE devices, do these phones require a continuous draw of 15.4 watts (W) (standard Class 3 allocation)?
A. No. When Cisco Unified IP phones are connected to common switches such as the Cisco CatalystÂ® 3560 Series Switches or Cisco Catalyst 3570 Switches, the switch can automatically detect the phone type and provide the exact amount of power that the phone needs. The use of Cisco Discovery Protocol provides this greater granularity to the power classification, allowing for more accurate reporting of power consumption, and a higher density of phones on the switch. The Cisco Unified IP Phone 7965G and 7945G models will draw a maximum of 12W from a switch; this places them beyond the limits of Class 2, but below the default 15.4W value of Class 3.
Q. Is there a recommended powering option for these phones?
A. No. Powering decisions depend on the customer's environment. The IEEE 802.3af inline power method can be used; alternatively, either the standard Cisco Unified IP Phone external power adapter or the Cisco Unified IP Phone Power Injector can be used for local power.
Thanks for the reply, not sure it relates to the issue I am having at the moment though. All our switches are the same model and the phone works fine on some of them just not all of the switches we have.
On the switches that it doesn't work on there are plenty of other 7945s working fine from those switches.
We recently moved this one phone during an office move and it's since then that it is picking about what switches that it will work on.
In addition to Rob's excellent post (+5 there mate), what specific model is your switch? The reason why I'm asking is because a number of people would buy, for example, a 48-port PoE switch and they never realize that you can only power 24-ports (15.4w) on a first-come-first-serve basis. So if you plug 25 7945 you only get 24 working and the 25th is dark.
And in light of this, some people get confused and say something like "but if I plug the contraption to another switch it works fine".
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