Yealink supports having different ringtones to differentiate between internal and external incoming calls. However, they require that this feature also be supported by the VoIP service, which is not ...
The steps are:
Click on "Directory"
Choose "Local Directory"
At this point you can create a group and call it "Internal" if you want to have a group for internal calls or just leave everything in "All Contacts"
Select the group where you want these contacts to go
Click Add
You can then enter the information for each user and the number will be their GoTo line number.
Other information --
If you add a group, then you can change the ringer sound for the entire group. You can also change it for each user if you wish but you can leave it on "auto" and it will follow the ringer sound of the group it's in.
If you don't create a group and put those users in the group then you'll need to make sure that you change the ringer for each user that you add.