If you run Android, that is a feature on recent versions (Marshmallow and up seems to have it - it might go older, I can't look).

I have actually put my phone into guest mode to lend it to people at the office.

My biggest concern if I let someone borrow my phone in a busy public space is that they will run off with it.

Perhaps the guest mode should primarily be a means of preventing a thief from doing anything meaningful with a stolen phone. It should probably restrict the 'guest' to only using the phone mode and prevent the phone from being placed into airplane mode so that if it is stolen it can more likely be tracked and recovered.

On recent android versions you can allow calling from the lock screen. So just lock your phone and switch to the dialer before handing it over.