Can we use "Application User" as the 'Owner ID' & 'Digest User' when Adding a 3rd Party Sip Phone? Since our CUCM is integrated with LDAP, we cannot create new 'End User' without calling the AD Administrator.
You need an End User account. Just create a new OU in LDAP and place all of the phones there if they are not assigned to a real user. Assign a long/random password to secure the account from all other logins.