You can use TempData.You can pass every types of data between to action, whether they are in same controller or not. Your code should be something like it:
public ActionResult GetMDN(string msisdn)
{
int sngid=10;
TempData["ID"] = sngid;
return RedirectToAction("IIndex");
}
public ActionResult IIndex()
{
int id = Convert.ToInt32(TempData["ID"]);// id will be 10;
}