Panasonic homephone KX-TG785SK keeps saying new battery is this for handset batteries?

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: I have a Panasonic KX-TG6051CM

I had the same problem, and it seems that the battery charging circuit in the handset had locked up, constantly charging the battery until it overheated. I was able to reset the circuit in my handset by completely draining the battery. Here is the procedure:

1. Remove the handset from the charging cradle, and leave it on for a few days until the battery runs out, and the handset finally turns off. At his point, the battery will be very low, BUT NOT COMPLETELY DRAINED.

2. Next, you must manually drain the charge remaining in the battery by shorting out the two contacts on it. Shorting won't damage the battery, since it is holding very little charge at this point. An easy way to physically do this is to remove the battery, and place a piece of thin wire across the the two contacts of the HANDSET. I am referring to the contacts inside the handset that connect to the battery, NOT the contacts on the bottom outside that connect the handset to the charging cradle. Next, put the battery back in, sandwiching the wire between the battery's contacts and those inside the handset. This will hold the wire in place. DO NOT PUT THE HANDSET BACK INTO THE CHARGER, WITH THE BATTERY SHORTED OUT.

3. Wait about 30 minutes for the battery to completely die. Remove the jumper wire, and but the handset back into the charger.

4. If the battery was fully discharged , you will get a message on the handset telling you that you should let it will charge for 7 hours. If you do not get this message, then either the battery was not fully discharged, or your problem is different from mine. Try the procedure again in Step 3 for another 30 minutes to discharge the battery some more.

5. If you get the "Wait for 7 hours to charge" message on the handset, then leave it in the charging cradle for the 7 hours requested. After that, it should work fine.

Let me know how you make out.

SOURCE: Handset voice very low and LCD display is blank

My 1740 handset volume was also low. As a shot in the dark I changed the speaker and it fixed the problem. I got a new speaker from www.panasonic.com .

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-tga101cs incorrect voicemail message

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