Why do they need a "lifeline"? They can ask an adult to call you if they need something. If you don't trust them at a sleepover or play date, then don't send them.
I let my kids go to their friends' houses all the time and sometimes to the local park by themselves, and I've never once regretted not giving them a phone. They know our address, phone numbers, and how to get home, and we pre-arrange what time they should be home (they have simple watches).