Regular Android. They are not going to be able to change the OS or install other things. It must be as easy as plug your old phone in, download an app and let it run.
Android does allow you to run background and foreground services of many kinds.
Discussion
Ah you’re assuming they have an android here… Yeah tmux is a good idea… But if they have iOS its def not as straightforward to my knowledge … its the stupid simple things like — I’m not sure iOS has allowed INCOMING connections on factory install since the maybe the second version of them.
No one uses iOS outside the developed countries. Everybody has an Android lying around.
And yes, iOS will never allow you to do this.