Can't sign up for Instagram on my iPhone 5?

[Becky]

I've been trying to sign up for Instagram on my iPhone 5 but when I complete the details page and hit done it says "The IP address you are using has been flagged as an open proxy. If you believe this to be incorrect, please visit http://help.instagram.com".

I can't find anything in that link to help me and have no idea what an open proxy is. Went round to a friends and used the wireless at her house (which needed a password) and it still didn't do anything. Took it to the apple shop and they couldn't help either.

Got a friend to create an account for me so I could skip the sign up process and just log in off my phone but it says the username/password is invalid, when we're sure it isn't.

Please help