Phone calls from heaven

So the craziest thing happened this past weekend. My sister recieved three phone calls from my dad's cell phone. His phone was deactivated months ago. She said when she answered no one was there, and then the number called her back twice more. Each time it came up on her cell phone as "dad mobile". It is so strange. Of course I am trying to research logical and scientific explanation for it, but do kind of like the thought of dad calling us from Heaven.