After reading Mobility guide from Microsoft, it is not very clear that how users will connect to Lync auto discover service externally and internally in split brain DNS Setup and it requires reading again and again and troubleshooting again and again to find out the correct setup, in summary following should be the DNS configuration in split-brain DNs setup (Note: Split-brain DNS refers to the DNS setup where your DNS zone is split between your Internal Network and your external network for example Contoso.com zone is also present in internal AD forest DNS servers and also published outside publish DNS servers);
1. Lync Auto discover URL should be in external and internal DNS servers pointing to Lync external web services URL.
2. Lync Auto discover internal URL should be present in your internal DNS server pointing to Lync internal web services URL.
3. There should be a entry for Lync external web services URL in your internal DNS servers pointing to lync external web services IP for “Hair-pining” Lync mobile traffic to Reverse proxy servers.
4. Reverse proxy Rule for lync auto discover service should have lync discover internal URL added for mobile clients connecting from internal network through lync external web service public IP.