Call Waiting (known in Japan as "Catch-Hon," a registered trademark of NTT) lets you put the current call on hold and answer another incoming call. On NTT landlines it is a paid monthly option: when a "beep-beep" interruption tone sounds during a call, you press the hook button to switch between the two calls. Monthly charges are revised from time to time, so check the current amount with your provider.
Without Call Waiting, anyone who rings while you are on the line hears a busy tone and cannot get through. For work where a call from a client must not be missed, the service was essential for decades. Mobile phones offer the same function, but the name differs by carrier - "Catch-Hon," "call interruption" and so on - and whether it works from the start or has to be applied for also depends on the carrier and the price plan.
To use it, press the hook button (landline) or the catch button (mobile) when the interruption tone sounds to answer the new call, then press again to return to the first one. Because the calls are switched rather than joined, three people cannot speak at the same time, and whoever is on hold is left waiting. For a conversation with three people at once you need a three-way calling service. Someone kept on hold too long may simply hang up, so it is good practice to say "another call has come in, I will call you back" before switching.
As of 2026, SMS and messaging apps let people reach you during a call, so there are more situations where the message can wait rather than answering the interruption. Some services built on VoIP include waiting-call notification from the start, while others treat it as a paid option, so it is not always bundled - check what your own contract covers. See the business phone number guide for more.