Since the network is aware of which device successfully answered the call, and also which devices have expressed an interest in the call, it is able to send a REL message to all those devices which sent an ALERT but did not answer. This stops these devices from ringing (or whatever is the equivalent activity for the type of device in question).

The interesting point about this scenario is that devices sharing the same line communicate with the network. They do not communicate with each other. In fact, they are unaware of the existence of each other.