Methods and apparatus for increasing security and control of voice communication sessions, such as Push-To-Talk (PTT) over Cellular (PoC) sessions, using digital certificates are disclosed. A certificate control mechanism of a private communication network is used to help facilitate secure PoC sessions for mobile communication devices associated with the private network. The certificate control mechanism of includes a certificate controller which is adapted to communicate with a Certificate Authority (CA) component which produces and stores a plurality of digital certificates for the mobile devices. The certificate controller is further adapted to communicate with one of a PoC server and a PoC group list server through a public communication network (e.g. the Internet) so as to provide the plurality of digital certificates for secure PoC communication sessions for chat groups which include the plurality of mobile communication devices. In one illustrative technique, a request message for participation in a PoC session is received by an authentication component of a PoC server from a mobile device. The request message is signed with use of a private key of a digital certificate which may be associated with the mobile device, an end user of the mobile device, or a chat group which includes the mobile device. An authentication procedure is performed on the request message by the authentication component. The authentication procedure includes retrieving a public key associated with the digital certificate and attempting to validate the signature with use of the public key. Processing for the PoC communication session is permitted for the mobile communication device if the authentication is successful, and otherwise denied. Several other techniques and variations are described.