I have one called "Fi". Works with your phone. Seems to work great but I don't use it that often. When I think there might be an issue I'll swap the regular collar out for it. If the dog was off lead I'd always have it on.
It has an alert and light on the collar so it can act as something of a beacon. It also counts how many steps your dog has taken and how far it has gone (no guarantees about accuracy).