The latter. If they don't get the "number not in service" answer, there's also the possibility of someone on the other end picking up who is nowhere close to the person they expect. The characters from a person's home world they are trying to reach would not exist within the world of Caligo, and thus if a character chooses to call that number in this world, there's the distinct possibility a stranger native to Caligo's world has and is using that number.