Currently, the only system which integrates directly with Access RDB is Cloudcall.
If you dial from Access RDB it will utilise the Windows Tapi dialler. If your phone system can pick up from that then it can be used.
This call is made from RDBDialler.vbs in the folder RDB is installed in. You could potentially edit this to make a direct call to your phone system but this is not supported by The Access Group support teams.