I am just researching this issue myself.
Apparently if you take a look at official site's php that generates the json for the examples, you will see that its not standard JSON.
It has some special format:
{
"page": 1,
"total": 2,
"rows": [
{
"id": "ZW",
"cell": {
"name": "Zimbabwe ",
"iso": "ZW",
"printable_name": "Zimbabwe ",
"iso3": "ZWE ",
"numcode": "716"
}
},
{
"id": "ZM",
"cell": {
"name": "Zambia ",
"iso": "ZM",
"printable_name": "Zambia ",
"iso3": "ZMB ",
"numcode": "894"
}
}
]
}
Apparently you are returning JSON format that has only the "cells"
You will need to find a way to create this structure in your returned JSON resultset.
I have found an example for ASP .NET MVC that converts it to the proper format : here