An analog line is a telephone line that converts voice into continuous electrical signals (analog signals) for transmission. Since Alexander Graham Bell's invention of the telephone in 1876, analog lines served as the foundation of telephone communications worldwide for over a century. They are contrasted with digital lines (ISDN) and VoIP.
In Japan, NTT's metallic (copper) lines are the representative analog lines. Each line supports only one call at a time, compared to ISDN (2 simultaneous calls) or Hikari Denwa (multiple simultaneous calls). Audio quality is limited to the 300 Hz-3,400 Hz voice band, producing the distinctive "telephone voice" with high frequencies cut off. Compared to VoLTE HD Voice (50-7,000 Hz), the analog voice band is roughly half.
The greatest advantage of analog lines is the ability to make calls during power outages. Power is supplied from the NTT exchange to the phone through the copper wire (line powering, approximately 48V DC), so phones that do not require an AC adapter can make and receive calls without a power outlet. This characteristic remains important as a disaster lifeline. However, cordless phones and fax-equipped phones that require AC adapters will not work during outages. Keeping one simple line-powered phone as a backup is recommended.
NTT migrated its landline network from PSTN to IP in January 2024. The main reasons were aging exchanges and rising maintenance costs. User-side phones and phone numbers remain usable, and call rates have been unified nationwide (9.35 yen per 3 minutes). However, line powering during outages may no longer be guaranteed after IP migration, making it important to have a mobile phone as a backup. As the transition to Hikari Denwa progresses, analog lines are gradually reaching the end of their era.