Using OBi110 with firmware 1.3.0 (Build: 2774). By inspecting the logs from my DNS server, I discovered that the OBi queries for both
example.com and _sip._udp.example.com. This is fine. The symptoms: when one SRV record exists but not the other, sometimes registration fails. The error when trying to register to a SP is "Retrying Register (server=0.0.0.0:5060)".
I worked around this issue by creating SRV records for both
example.com and _sip._udp.example.com so that whichever one the OBi arbitrarily decides to use, it will work.
Based on my preliminary testing, the results seem to be the same (sometimes it uses the result of the prefixed name, sometimes it doesn't) regardless of the X_DnsSrvAutoPrefix setting.