You are officially allowed to make private calls from your work phone. But before you can do that you need to fill out the following form that allows the university to substract the costs directly from your payslip: https://www.uibk.ac.at/zid/abteilungen/netzkomm/telefonie/intranet/gehaltseinzug.html

To make private calls just add * in front of the phone number when dialing. You can also check your current costs in the VIS. For costs: https://www.uibk.ac.at/zid/abteilungen/netzkomm/telefonie/intranet/tarife_festnetz.html

You can also call through your work phone from every where: http://www.uibk.ac.at/zid/abteilungen/netzkomm/telefonie/callthrough.html

The CallTrough service also supports SIP. You can add a new SIP via the Phone Webinterface: https://phone.uibk.ac.at/