Hi Xavier,
I have always found this hard to explain, but I will try :)
Cisco PoE switches are not all created equal. Older models like the 3524-PWR XL that you are using only support what is called "Cisco Pre-Standard PoE" and do not support the IEEE Standard which is referred to as IEEE 802.3af PoE.
Now for the fun part?? Different models of Cisco IP Phones have different PoE requirements sometimes called "Classes" These Classes require different amounts of power supplied by either "Cisco Pre-Standard PoE" or IEEE 802.3af PoE. New Models of Cisco switches are backwards compliant to support both PoE modes but Old Models like the 3524 only support the one Pre-Standard mode (confused yet?)
So when using the Old models like the 3524 you always have to be careful of what IP Phone type you can or can't support.
Have a look;
The 7940G/7941G and 7960G/7961G are class 2 devices (7.7 watts) power requirement.The 7941G works with Cisco Pre-standard PoE and with IEEE 802.3af PoE.
From this doc;
http://www.cisco.com/en/US/customer/products/hw/phones/ps379/products_data_sheet0900aecd802ff012.html
The 7941G-GE/7961G-GE/7970G-GE phones are class 3 devices (15.4 watts) and only support IEEE 802.3af PoE.These are not configurable settings.
From this doc;
http://www.cisco.com/en/US/customer/products/hw/phones/ps379/products_data_sheet0900aecd802fefd4.html
Hope this helps!
Rob
