That will hugely depend on the phone model if they got it available through their configuration files. Remember that every phone manufacturer has a different configuration file that they manage.
I would recommend to check on additional options for your particular phone model’s configuration file. Now, I would also recommend the Multi-Cast RTP if you are dealing with SIP based PA systems. For normal extension I find it easier just to add the extension numbers. There is no need to do any configs on the phones this way.