From IOS 12.1(9)EA1 there is a new approach to connecting Cisco IP phones implemented in Cisco 3550switches. This closely resembles the Auxiliary VLAN on CatOS switches, it's called
Voice Vlan and it does not require the port to be trunking. So I would suggest to test the config exactly as described in the below link, that is leaving the port as access port and configuring Voice Vlan. This would leave portfast enabled and will not introduce trunking which seems unnecessary - only the voice vlan traffic will be tagged and the data vlan will be untagged ( as native vlan on dot1q trunk ). Requirement to this setup is to run 12.1(9)EA1 or ( better ) later.
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/1219ea1/3550scg/swvoip.htm
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/1219ea1/3550scg/swqos.htm
http://www.cisco.com/warp/public/cc/pd/si/casi/ca3500xl/prodlit/cfig3_an.htm
