Teaching pre schooler to make emergency calls

I am a lone parent to a 3.5 year old boy. We don't often get visitors at our home and one of my biggest fears is something happening to me and my son being left alone in the house with me incapacitated/ unconscious/dead... There's no risk of me being hurt at all but if I fall down the stairs or something I mean.I worry so much that he'll be alone in the house, scared and no one will come. How can I teach him to use my phone if mummy won't wake up or something?My phone is locked and we always have doors locked so he wouldn't be able to get to a neighbours house. Is there an app he'd be able to access from a locked iPhone or a technique anyone has used to teach a child to only call in an emergency/unlock phone ( I worry about giving him open access to my phone unless it's an emergency, he is only 3 and won't know not to take advantage of having the access)..I'm not being the most articulate so I'm sorry about that but it's a difficult subject to write about without coming across as anxious or morbid. Would be grateful for any advice via /r/Parenting http://ift.tt/29cbb2e