Or you could simply go to this site: http://voxilla.com/tools/device-configuration-wizard/
which allows you to configure any of your Linksys devices as well as towards the top of the list you'll see Linksys Certificate Authority Service for VoIP Adaptors which will generate and install the certificate on your adaptor/phone. You may have to register with Voxilla to use it but.
HTH,
\R