Problem is often the competence of people handling these proxies. Yesterday I discovered that my ISP is blocking all DELETE http calls (ACT fibernet in India)!

And before that they decided to enforce a reverse DNS lookup so all the name based virtual servers with local DNS entries stopped working - I had my staging instances setup that way.

And not only all this happens without any prior information, it is next to impossible to climb through the support layers to finally find someone who even understands what are you talking about. They just want you to restart your modem to "resolve the problem".