The other option is to update the PBX settings, so it assigns the same COS used on a working extension to the ones which currently cant make outbound calls.
For this you need to go into the programme mode which is done as follows (and described on page 43 of the manual here), please note you will need to know the PIN this by default is 1234 but most likely has been changed by who ever manages the telephone system.
From a phone model DT321 and the handset on the hook, press the program button then ** the PBX PIN (default being 1234), to enter COS dial 602, then press the autodial\store button, enter the extension which needs to be given outbound calling permission, enter the COS required probably 01 (but could be anything between 01 and 64), press autodial\store
This is a rather complicated process, if you run into issues after entering in ** at the start, it because the phone isnt granted admin rights and try and do the same from a managers or reception office phone.
Let me know how you get on.