It is known that by now many VoIP related services do not offer encryption of the data. Thus in theory it is not difficult to monitor the VoIP calls and even moderate their content. The ways of securing the phone conversations differ from company to company.
Some use patented audio codecs (hardly available for open source programs), but this method hasn’t proved to be effective. The compression used by other companies makes wiretapping more complicated. But to speak of a real security we should speak of encryption and cryptographic authentication – the notions unfortunately by now almost beyond ordinary customer’s reach.
Analog Telephone Adapters and different softphones offer SRTP and ZRTP protocols. P2P VoIP is possible to secure by opportunistic encryption of IPsec. The encryption clear to the provider is used by Skype.
Using IPSec encryption on the digital voice stream guarantees protected voice calls for business VoIP networks.