"When you enable digest authentication for a phone, Cisco Unified Communications Manager challenges all requests except keepalive messages for phones that are running SIP. Cisco Unified Communications Manager uses the digest credentials for the end user, as configured in the End User Configurationwindow, to validate the credentials that the phone offers"
You should not need digest authentication for CSF devices unless you want to add that extra layer of security, they are commonly required by 3rd party SIP endpoint registeting to CUCM.
