Put your phone into 3G/EDGE mode (not wifi) and try to open webmail in safari. If that works, it is ‘internet accessible’.
Additionally - when you get to step 16 below - the URL the script says is the ‘Server URL’ MUST be internet accessible. If you aren’t sure, open safari on your iPhone while in EDGE/3G mode and try to access it. If it says can’t find host - this is NOT an internet accessible URL.
You must have designer access (which is pretty normal)
If you’re not sure if you have this just go to the setup - if you don’t have designer access you won’t be able to proceed past step 5.