I have a few 525G2 units running 7.5.3 and they are provisioned using .cfg files located on a http server, i have no real issues but a few months ago i came across a problem where none of the phones would process the .cfg files.
I located the reason for this after a lot of head scratching and it was down to the dial plan, i have my dial plan set to use <:01792> as i don't like entering my local area code everytime i make a local call.
It appears that entering <:01792> as part of your dial plan directly into the phones web GUI works just fine, but if you add it as part of the dial plan in a .cfg file it kills provisioning! For some reason the tags < and > are the problem.
It seems as a result of being able to use < and > i have to use HTML character encodings in their place, so instead of adding <:01792> to my dial plan in the provisioning file i actually have to add <:01792> instead of <:01792>