The Panasonic KX-TGMA44AZB Extra Handset is compatible with the Panasonic KX-TGM420AZB Amplified Cordless Phone and the Panasonic KX-TGM422AZB Amplified Twin Cordless Phone. Both these phones are expandable up to 6 handsets and feature amplified incoming voice calls to support mild and moderate hearing loss along with Slow Talk control to automatically slow the speed of quick-talking caller voices while conversing in real time.