The KX-TGA401 is a handset for specific Panasonic cordless telephone systems. Connecting the handset requires registering (pairing) it with the base and if the base can support another handset. First make sure that the base is plugged into an active phone line (land line) and that the power adapter is plugged in and working properly. Charge the handset before attempting the registration process.
Then make sure that the phone system is in standby (no calls). Press and hold the Locator button on the base for about 5 seconds (wait until the tone sounds). If all of the handsets that are already registered ring, press and release the Locator button and then press and hold it again. Once you hear the registration tone (some models), place the new handset in the charger on the base. Wait for the long beep and the registered symbol to appear.
If the not registered symbol appears, try registering by using the handset menu. Press Menu # 1 3 0 (the key below 9 and individual number keys) on the handset. Press and hold the locator button until you hear the registration tone as above. Then press Select and Save on the handset within 90 seconds of hearing the registration tone. Wait for the handset and base to complete the registration process. You'll hear a long beep.
The first registration method is given in the manual for the handset, http://help.panasonic.ca/viewing/ALL/KX-TGA401/OI/TGA401_PNQX2390ZA/TGA401_PNQX2390ZA.pdf
. The second is from the page 31 of the manual for the KX-TG653x (KX-TG6531 base) system, ftp://ftp.panasonic.com/telephone/om/kx-tg6511_mul_om.pdf
. The KX-TGA401 handset works with other bases as well. You may want to check the manual for your phone system in case there are differences. Other compatible phone systems are the KX-TG401x, KX-TG403x, and KX-TG651x.
I hope this helps.
(I make sure that the land line is active by checking for dial tone with an old corded phone.)