You can "collect" dwellers in multiple ways: via Lunchboxes, a Radio Room, or by breeding.
Now, I know that when a woman gets pregnant, she won't deliver the baby unless you choose to.
But what about dwellers from outside the Vault? Do they leave after a certain amount of time? Is it the same for dwellers from Lunchboxes?
What about collecting a call from the Radio Room? If I don't collect the call, will that call vanish after a certain amount of time?