Visual voicemail has access to a visual interface, which shows the list of playable voicemail messages and sometimes can include decryption of those messages.
Inter-Asterisk eXchange (IAX) is a communication protocol used to initiate user sessions, similar to an SIP, in order to transmit and control streaming media, especially in Voice over Internet Protocol calls. IAX2 (the newest version) is a preferred method because of its flexibility, as it is compatible with a variety of codecs, and jitters and lag are minimal due to trunking and multiplexing, which means the amount of bandwidth and latency are also kept at minimal levels.