RFC 4239 Internet Voice

For some forms of communication, people prefer to communicate using their voices rather than typing. By permitting voicemail to be implemented in an interoperable way on top of Internet Mail, voice messaging and electronic mail no longer need to remain in separate, isolated worlds, and users will be able to choose the most appropriate form of communication. This will also enable new types of devices, without keyboards, to be used to participate in electronic messaging when mobile, in a hostile environment, or in spite of disabilities.