Call transfer automatically forwards calls placed to your number to another number you register in advance. On fixed lines, NTT East and NTT West offer it as a paid monthly option called "Voice Warp"; on mobile, Docomo offers "Tenso Denwa Service" and au offers its "Call Transfer Service." Neither mobile service charges a monthly fee, but how you start using it differs by carrier: Docomo requires you to sign up first, while au needs no application - you just turn the setting on (as of August 2026). Because charges and terms are revised from time to time, check the current amounts on your own carrier's official pages.
Three forwarding modes are common. Unconditional forwarding (au calls it "full transfer") sends every incoming call straight to the other number. No-answer forwarding kicks in after a set number of rings without pickup. Busy forwarding applies when another call arrives while you are already on the line. Carriers use different names for the same modes, and some combinations are mutually exclusive: on au's Call Transfer Service, full transfer cannot be set at the same time as no-answer or busy forwarding (as of August 2026).
Business uses include forwarding to sales staff who are out of the office, routing after-hours calls to an answering service, and reaching employees working from home during telework. Combined with PBX extension transfers, it builds a path that reliably connects a customer to the right person.
One caveat: the call charge for the forwarded leg is billed to the subscriber who set up the forwarding, not to the original caller. Forwarding heavily to mobiles or distant numbers therefore adds up. Forwarding can also be mutually exclusive with voicemail - on au, starting call transfer stops the "Voicemail Service EX" option. And if you forget to turn forwarding off, calls keep going to a number you no longer intend. Learn how to cancel the setting before you rely on it. See the business phone number guide for more.