1 Let the IP Phones use switch as the only power source or disable inline power on the switch and let the external power source be he power source.
2 Always Power up the IP Phones before powering up the switch. When an IP Phone is connected to an inline power capable switch, the switch will send a signal to find out if the phone requires inline power or not and expect the signal to be looped back to Itself. If the ip phone is not powered up before switch would provide inline power and later when the Ip phones are powered from external source it would result in power conflict. Therefore in order to prevent error IP Phone should be powered up before connecting to the switch.
3. With the Catalyst 3550 24 PWR, up to 15.4W/port of inline power on every 10/100 port ensures maximum device support for IP telephony and wireless LAN deployments.
This URL should help you:
http://www.cisco.com/en/US/products/hw/switches/ps646/ps4882/index.html
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/1219ea1/3550hig/35trbl.htm
