You mean that a user who is not a domain admnistrator should be able to log in to the Univention Management Console (the web-based management tool), edit another user and change his/her telephone number?
This should be possible. The UCS documentation contains a section (“3.4.5. Access control for the LDAP directory”) about allowing certain users to reset the password of other users. You should be able to map that approach to allowing them to change the telephone number. The section “4.9. Delegated administration in the UMC” is relevant, too.
Additionally there’s a Cool solutions article about this topic which might be easier to understand than the admin documentation.