There have been various efforts by industry groups like the SIP Forum and individual vendor initiatives like Sylantro usually results in competitor comments about the implementation being proprietary, etc. The back-and-forth understandably keeps going on. This ends up frustrating customers and end users.
There is hope finally! The SIP chairs have now decided to tackle this services interoperability issue head-on by getting together a BOF called BLISS. The name is apt and I would strongly recommend that any of you who are in the industry support this initiative and actively participate in the discussion.