The charcoal gray AT-406B Indoor Handset-to-Handset Audio Only System from Aiphone is designed for two-way communication and consists of one AT-206 Master and one AT-306 Sub Unit. Up to 2 additional subs (not included) can be added to the system.

The call button on the master alerts all subs in the system, whereas the sub alerts only the master unit. Both the AT-206 and AT-306 can be powered by 4 AA batteries or 6 VDC power supply (both not included). Any particular station can also be signaled by a bell or a light, while the handset rings using a TAR-3 relay (available separately).