Some users prefer to have different ringtones in order to differentiate external calls from internal calls. Another usage would be to set to use the "Silent" ringtone only for calls from external numbers.
To use different ringtones for external vs. internal calls there are two possibilities.
You can set different ringtones for certain contact groups in the phone's Directory.
Previous FW version: Under Identity → Ringtone, set the Ringtone to the ringtone that you would like to have for external calls:
The second option must be supported by your PBX/phone system. The PBX can send the SIP Alert-Info header within the SIP INVITE method sent to the phone. In this header, the PBX can mark what kind of call it is. Here is an example of the SIP Alert-Info header in a call that is marked as external:
Similar for internal calls:
Then, you can select the desired ringtones on the phone under Preferences -> Audio -> Alert-Info Ringer: