no, line and device css are concatenated, and depending on how the systems is setup (traditional or device/line) you might want to change the line css instead of device css
it's good to understand how your environment is setup, the SRND has a good chapter on it;
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/8x/dialplan.html#wp1150926regards
Sascha