Another Internet application which lends itself for integration with telephony is email. A traditional PSTN application which can be replaced with VoIP and email is voicemail. Traditional PSTN voicemail systems feature fairly inconvenient user interface for message retrieval: IVR (Interactive Voice Responder). Callers have to navigate through an automated voice menu, listen to lengthy announcements, type digits as prompted and be very patient to achieve very simple tasks. It is undoubtedly more convenient to deliver recorded messages to called party by email. The called party can then listen, store and process the received messages at his convenience. The following picture shows the data flow in a voicemail-2-email scenario. An open-source voicemail2email application, SEMS, is available from iptel.org.
SEMS stands for SIP Express Media Server. It is an application framework that allows easily to build applications that deal with media streams. The framework itself provides only minimal functionality for accessing and manipulation of media streams and signalling. High level logic is stored in additional modules that can be dynamically loaded. Examples of such modules are voicemail, announcement server, and ISDN gateway.
Some other voicemail systems do exist, they include OpenAM which is available from the OpenH323 website and a voicemail system built-in inside the VOCAL system. Unfortunately they are not easy to set-up and they are not yet ready to be used in a production environment if you need a completly integrated product, anyway they are working without bugs and are simple to be customized for small environment scenarios.