How do I cancel 'on hold' on my Panasonic KX-TG8521E handset?

SOURCE: Panasonic KX-TG2740 Cordless Phone - Can I reset handset registration

press function button on handset, scroll to initial setting, select by pressing right arrow, scroll to registration, select, select hs registration, follow directions on handset display, pressing mute on "new" base station, and ok on handset(plugged in). If you like, you can unregister with the "old" base unit first.

SOURCE: adding handset to Panasonic KX-TGA542M phone. ? how to ?

I ordered a new kx-tg560m handset for my kx-tg5632 base unit, but the enclosed instructions for registering it with the base unit were wrong. Fortunately, I found the old quick setup guide in my files.

If you do not have the same base unit and handset, go to the URL:
http://www.panaxonic.com/RegisterYourHandset.

1. On the handset, press [MENU], then press [0][0]. The handset displays "Refer to manual to prepare base. Then press OK->"
2. On the base unit, press [SELECT] until the desired extension number [H1, H2, H3, or H4] is displayed on the base unit, then press and hold [CALL] until the CHARGE indicator flashes.
3. On the handset, press the [OK] softkey.

When the handset is off and sitting in the charger, the handset's extension is displayed in the upper right corner.

SOURCE: Panasonic cordless handsets all say line in use

I had the same problem. Something makes the phone's memory get "stuck."  Take the battery out of the phone for a minute and then put it back. Then unplug the phone and plug in again. You need to do this with all extensions. The problem goes away. 

SOURCE: Panasonic KX-T7730X on hold music ringing non-stop

The solution is press 1 on hook condition. this is Back ground music .

SOURCE: Panasonic kx-tga101cs incorrect voicemail message

Press and hold "Clear" until it beeps. It will clear the invalid voice mail msg.