Draytek Vigor2860 User Manual page 316

2. Even no voice prompt, but the RTP traffic is still secured until the call ends.
Info If the incoming or outgoing calls do not match any entry on the phonebook,
the router will try to make the call "being protected". But, if the call ends
up "unprotected"(e.g. peer side does not support ZRTP+SRTP), the router
will not play out a warning message.
302 Vigor2860 Series User's Guide