Adding to your question, it seems there are two things that are needed: Reducing reliance on a third-party service (phone company, Skype, etc.) that may keep call records which can be subpoenaed; and encrypting the conversation itself so it can't be monitored.

It seeems there are "crypto phones", but do these still rely on a third-party service to initiate and transmit the call? http://en.wikipedia.org/wiki/Crypto_phone