Connecting “Call Waiting”, you will not miss an important call, even if you receive it during a phone conversation! 
A specific tone signal will inform about second call notification. You can make a choice whom to talk to first and speak in turn to each telephone interlocutor. 

“Call Waiting” service allows the subscriber to accept another call during a phone conversation. If another interlocutor calls the subscriber during a phone conversation, a specific tone signal will ring. The subscriber, who receives the call, can:

• continue the current telephone conversation without paying attention to signal
• interrupt the current telephone conversation and answer the call
• answer the call and then speak to each interlocutor in turn, using the complex of Call Waiting/Call Holding service

“Call Holding” Service also allows calling another subscriber during a telephone conversation, and then speaking to each interlocutor in turn¹.