Provisioning is so weirdly diverse. http, t*f*t*p, https, DPMA; XML, INI, proprietary formats; relying on various HTTP headers; the list goes on and on.
Doing EPM for multiple vendors is crazy. That Andrew did all of this in the past amazes me. It also annoys me greatly because my next thought is "it shouldn't have to be this way."
TR-069 is implemented by some phones but it's hardly a well-accepted standard for VoIP endpoints.
I think the way forward for IncrediblePBX or any project that wants to add provisioning is to standardize on one or two endpoint vendors and just get those perfect. And stick to your guns on it, because there's always someone coming along "Hey why isn't my Cisco Selsius 7910 supported?" that needs to be told it ain't going to happen.