For Vtech LS 6245 you cannot delete all new messages, but can delete all old messages. So the fastest way to delete all messages that are new is to
1) Listen to messages
2) Skip them all with the forward button, this can be done faster than deleting each individual one
3) Go in to the answering machine menu and delete all old messages, it will now delete all the messages you just skipped :)
Yoms