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:
Add the internal numbers to the Directory (manually or by importing csv)
You can set different ringtones for certain contact groups in the phone's Directory.
1. under Directory → Add or Edit Entry, add all internal numbers as new contacts with Group: "Colleagues"
2. Under Identity → Ringtone, set the Ringtone to the ringtone that you would like to have for external calls:
3. Under Preferences -> Audio -> Directory Ringtones -> Colleagues, set the ringtone that you would like to have for internal calls (calls from colleagues):
Woth this setup: if a colleague calls, the phone will use Ringer 4, otherwise it will use Ringer 1.
If you have too many colleagues to enter all numbers manually, you can also import a .csv list.
Automatic feature on PBX
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 ring tones on the phone under Preferences -> Audio -> Alert-Info Ringer: