When I tried entering the url into my browser (with my useragent set as an iphone), I got that same response. I managed to get a correct response when I used the live http headers extension for firefox, and replayed a recording of dialing the number using the iphone web app. The post content listed in the Live HTTP Headers extension is: Code: {"gvx":"random-letters-and-numbers-here:thirteennumbershere"} EDIT: I figured out what the post data is. This is the content of the cookie named gvx. Were you submitting the url as a post request, or a get request? I can attach the Live HTTP Headers file if that would be helpful.
{"gvx":"random-letters-and-numbers-here:thirteennumbershere"}