you can query the UCS LDAP for properties of users, and you're free to convert them into entries like
exten => 1234,1,Dial(SIP/the_user)
which you can include into your extension.conf. Along with entries of the form
in the sip.conf file, this would allow users to use any SIP-capable application (softphone) to be called and to dial (as you have defined the permissions in the given context).
The possibilities are endless... I'd suggest you to read at least asteriskdocs.org/en/3rd_Edit ... 91590.html and to pick what's suitable to your particular setup.