Currently i am using the following methods. I am not sure if this is the correct way to identify each unique phone (doesn't need to have same sim card). For android there is a imei phone
public static String GetDeviceUniqueID()
{
object DeviceUniqueID;
byte[] DeviceIDbyte = null;
if (DeviceExtendedProperties.TryGetValue("DeviceUniqueId", out DeviceUniqueID))
DeviceIDbyte = (byte[])DeviceUniqueID;
string DeviceID = Convert.ToBase64String(DeviceIDbyte);
return DeviceID;
}