Suteki na Kataomoi (1990)

Shunpei sometimes sees his neighbor Keiko, but doesn't think much of her. One of Keiko's friends gives her the phone number of a single guy, but when she calls she recognizes him as Shunpei, and gives him the made up name 'Nana.' When they meet face to face, she pretends not to know him, but through their phone calls, they slowly start to fall in love.