call waiting

telephone service which notifies users that there is another incoming call

Call waiting, in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new incoming call, and can then negotiate with the new or the current caller an appropriate time to ring back if the message is important, or to quickly handle a separate incoming call.
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Noun
1. a way of letting you know that someone else is calling when you are using your telephone
(hypernym) telephone, telephony

In telephony, a service feature that provides an indication to a terminal already engaged in an established call that one or more calls are awaiting connection.