If you want to use Power over Internet (PoE) to run your IP phones, make sure they're compatible with your switches. Before the 802.3af standard for PoE came out, Cisco had its own "pre-standard" version of PoE, which it originally called "inline power." Many Cisco phones will work with both the IEEE 802.3af standard and the older pre-standard version. Furthermore, the switch should automatically detect what protocol to use.
However, there are a few models that only accept the 802.3af standard. For example, according to Cisco, IP phones in the 7900 series that require 802.3af include:
The phones with "-GE" on the end support Gigabit Ethernet but not pre-standard PoE, whereas some of the non-GE versions (e.g., 7961G, 7941G) support both PoE protocols. This is presumably because the pre-standard PoE version predated Gigabit Ethernet.
Fortunately, some users have found workarounds for using some of the newer phones with older switches. The following command has been reported to allow 7961G-GE phones to work with 3550 switches:
inline delay shutdown 20 initial 300