Here is the answer:
I changed the Instance name of GetDeviceDataResult to the "GetDeviceDataResult" in ResponseData class. Previously I had this as deviceResult
public class GetDeviceDataResult
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string Id { get; set; }
}
public class ResponseData
{
public GetDeviceDataResult GetDeviceDataResult { get; set; }
}