ACS offers a stable and usable telephony voice response server for GNU/Linux systems today.
ACS, also known as the GPL licensed "Adjunct Communication Server" is a public project started to develop the core class extensible threaded C++ state engine and IVR component of a complete and commercial quality multi-line communications server. Combined with appropriate utilities, ACS can provide extensible IVR and messaging functionality. Many other elements include existing services that the ACS and related utilities can interact with. These will include Apache for web content, sendmail/ipop/etc for e-mail content delivery, openldap for directory services, and embrola for TTS. ACS can eventually be deployed in commercial settings as the primary resource for fax, IVR, voice mail, and unified messaging.