[VOIPSEC] RTP or SRTP inside UDP - how understand?

[Sergey Vointsev]

> The more pressing question would be: if you're receiving an SRTP stream,
> why don't you know about it?

Of course I know.
But I heard somewhere, that if some gateway in the net receives
something, that it considers to be RTP packet with some known codec
used, it can transcode it to some other codec. Am I misinformed? (yes,
I'm novice to VoIP :)
So actually I want to know how can we tell such devices "payload is
encrypted, don't touch it!".