Do you want to erase all messages? Are you looking at the handset, the base or are you calling the KX-TG885SK remotely?
1) At the base, press Erase during playback of a message to delete the current message. To erase all messages from the base, press Erase twice when the phone system is in standby mode (no active calls/incoming calls).
2) From a handset, use the menu to access the Answering Device sub menu and choose play new message or play all messages. (Alternatively, press Menu # 3 2 3 to play new messages or Menu # 3 2 4 to play all messages.) During playback of a message, press * 4 or pause the message. After pausing the message, highlight Erase and press Select. Confirm this. To erase all messages, choose Erase All messages (Menu # 3 2 5) and follow the prompts to confirm (highlight Yes and press Select). You can also press * 5 to erase all messages from the Answering Device sub menu.
3) During a remote call, *4 and * 5 work as above when using the handset.
See pages 70, 71, and 74 in the operating manual. The manual is available on the Add-on Handset page: https://shop.panasonic.com/cordless-corded-telephones/telephone-handsets/KX-TGFA61B.html
. The current link to the manual is http://help.panasonic.ca/viewing/ALL/KX-TG885SK/OI/TG885SK_PNQP1397ZA/TG885SK_PNQP1397ZA.pdf
I hope this helps.