According to the New York Times, it appears as if consumers in Italy are rapidly moving toward encryption for voice technologies due to rampant publication of private conversations, both due to leaked conversations that were a result of government wiretaps as well as conversations recorded through private means. From the article:
What has spurred encryption sales is not so much the legal wiretapping authorized by Italian magistrates–though information about those calls is also frequently leaked to the press–but the widespread availability of wiretapping technology over the Internet, which has created a growing pool of amateur eavesdroppers. Those snoops have a ready market in the Italian media for filched celebrity conversations.
It would seem that in Italy, it’s fairly common to take someone’s private conversations straight to the press… Even the national telco’s head of Security was in on the game:
This year, Bonini’s name was among thousands that surfaced in an illegal-wiretapping scandal involving employees of Telecom Italia, the Italian phone company.
Twenty people were arrested, including the former chief of Telecom Italia security, in what investigators say was an attempt to use the intercepted phone conversations to blackmail Italian public figures.
Many of the cell-phone encryption products mentioned in the article that are being marketed to Italian consumers sound a lot like Zfone, essentially providing end-to-end encryption for the audio between two devices that run the encryption software in advance of the call.
These devices from Snapcom and Caspertech generally use a circuit switched data channel (CSD) over the GSM network. Sessopn key generation and exchange most likely uses Diffie-Hellman over a proprietry messaging protocol. As you mention the encryption is done at the end points of the call, with no involvment of any third party or PKI. There are a number of other companies out there with similar products including Global Mobile Encyption, Cryptophone and Secure GSM.