As eil points out, cell phone calls are heavily compressed with lossless compression specially suited for the human voice (even DTMF codes get mangled; just to give an idea: a gsm cell phone compresses its audio track to a 13 kbps stream (FYI: an mp3 file with sub-cd quality audio requires 128 kbps for stereo sound)).
But as a first concept, if delay is not really a problem: reversed speech is not clearly understandable and might make it through the compression. Though it's not as good as real encryption, it's something that might work (I haven't tested it). And perhaps even a bit better delay-wise: splitting the audio-stream in chunks (of a few hundred ms or even a few seconds), can lower the delay but I guess the shorter each chunk of reverse audio, the better it could become to understand everything.
For shorted messages, it might work to just send audio files through MMS; and those audio files can of course be altered in any way you want, I guess.
So the most secure option would be to use VoIP, which can be encrypted without problem.