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 ...
Another thing that you can do is to create a directory of the internal user and then you can give the directory entry its own ring sound. I've done that for small offices. It's a little more difficult when it's a large customer.
I started doing that and it was working great...until.. someone changes and now the name is wrong