I've been on two deployments of NAC where the customer had Shoretel phones. The phones are setup by default to have 802.1x turned on, for whatever reason. Also, there isn't a magic button in their mgmt software to turn it off, nor a config file you can use (although Shoretel does mention that as an option, we haven't gotten it to work) . It has to be turned off on each phone before you can get them to MAC auth, since 802.1x takes precedence over MAC. Here is one link to turn off 802.1x on the phone, not sure if all phones are the same, you may have to consult with your phone vendor to find out the key sequence to turn it off.
Shortel Config Setup.