TCP extension

Anybody know how to setup a TCP sip extension? I want to connect my Nokia to Asterisk via TCP. This supposedly saves a lot of battery compared to UDP.

I am running FreePBX 2.8 with Asterisk 1.6 which is TCP capable. My sip.conf file contains

;to enable TCP support


Is tcp support only available on the trunk level but not the extension level. Does anyone have this running? I would appreciate any help. Thanks.