The dual number service allowed subscribers to have a second telephone number and to only receive calls made to this second number. The subscriber could then only let a limited number of people such as friends or relatives know this second telephone number. The telephone number change service enabled people worried by wrong numbers or nuisance calls to request to have their number changed. Both services were introduced to provide a means of defense for people worried by nuisance calls such as malicious or prank calls. The service operated on a trial basis for three years and was officially introduced in 1985 (Showa 60).

---