A nice work around I found in the Yealink forum, see option #2. I have had no luck with this version and the T48G/S getting distinctive ring to work with inbound routes, and thought I was out of luck with getting internal calls to ring distinctly. Posters suggested having inbound calls ring distinctly and leaving default ringtone for internal calls. This uses the phones directory to set distinctive ring for extensions, not sure how it would treat inbound DID's. You can export the directory xml file once you create it on one phone and just import to other phones, importing brings along the distinctive ring you select on the first phone.
http://support.yealink.com/faq/faqInfo?id=1
http://support.yealink.com/faq/faqInfo?id=1
Phone System 