...
The device configuration file will be the MAC address of the phone with a .cfg extension at the end. The Phone will request the common file first and then the device configuration file second. As mentioned before you want to to populate the common file with settings that all phones will utilize while populating the device configuration file with device specific settings such as the phones pjsip account credentials.
A20, A22, and A25 TXT Configuration
A30 TXT Configuration
For a basic PJSIP line configuration to be added to your device configuration file you should review the SIP CONFIG MODULE Elements within the text configuration file at the link above.
...