When/ how to teach child to call 911

I just realized I have not taught my daughters how to do this in case of an emergency. They are 3 & 4 but turning 4&5 and now I feel awful and really behind. No one plans emergencies but I want them prepared so they can be calm and do what they need to to get help. What age do you start teaching this and how? I don't have a landline, I do have a cell phone but there is no lock on it because my youngest has accidentally called emergency services from the lock screen when she was 2. Do I put the lock back on and show them that way? How do I explain emergency situations ? I don't want to scare them with situations, but I also don't want them calling because they got a scratch or they can't find their favorite toys and it feels like an emergency to them. Any tips and techniques? Thank you in advance!