Er... what's wrong with a good-old telephone ... while there is a record of the call being made (at the telco), the talker can (optionally) request the listener authenticate him/herself, there's no trace of the contents (the conversation) as soon as its received. The talker can also request and obtain immediate confirmation that the message has been received and understood.
And if you're worried about the record of the call, use someone else's - or a phone box.
Even better, meet in person.
Sounds like a better solution...