The KX-TGA401C handset works with the KX-TG401x, KX-TG403X and KX-TG651x phone systems among others. If you have the appropriate base, see the suggestions below.
Does your handset read Ringer Off? Try setting the ringer volume. With the phone system in standby (no calls in or out for any handset), press Menu. Navigate using the Up/Down arrows to Initial Settings. Press Menu/OK. Navigate to Ringer Settings and press OK. Press OK again to enter the Ringer Volume sub-menu. Use the Up/Down arrows to set the ringer volume and press OK to save. Press Off to exit the menu system.
The other possibility is that the handset is not registered to the base. Note: only 6 handsets can be registered to the base. If you previously registered 6 handsets, you need to deregister some of them.
To register the handset, hold the handset near the base since you will need to press buttons on both. Press Menu on the handset and navigate to and enter Initial Settings. Navigate to Registration and press OK. Navigate to HS Registration and press OK. On the base, press and hold the Locator button for 5 seconds and the registration tone sounds. Press OK on the handset you are trying to register. If all the registered handsets start ringing, you triggered the Locate handset function. Press the Locator button again. Then do the press and hold step again. Only press OK on the handset when you hear the registration tone.
You can find the manual for the KX-TG4011 here: https://shop.panasonic.com/support-only/KX-TG4011N.html
. The direct link to the manual is https://ftp.panasonic.com/telephone/om/kx-tg4011_mul_om.pdf
. See page 21 for the directions to set the ringer volume.
I hope this helps.
(You can't register a handset to an incompatible base. The communication protocols don't work.)