Asterisk is a hybrid TDM and packet voice PBX (Private Branch eXchange) and IVR platform with ACD functionality. It acts as middleware between the Internet (IAX, SIP, MGCP, Skinny, H.323), telephony channels (like Zaptel, T1, PRI, E1, FXO, FXS, VoIP, VoFR, ISDN, modems, Internet Phone Jack, etc.), and applications (like voice-mail, conferencing, directories, MP3 players, intercoms, etc.). It has many advanced features such as a codec translation API. The base distribution includes several channel backends, as well as applications. However, the beauty of Asterisk is its ability to be extended using its APIs, dynamic module loader, and AGI scripting interface. End users can even write their own applications that run on the system in C or any scripting language of their choice.
El Cid is a caller ID program for Linux. It just sits on your modem waiting for something to happen, parses the input, and displays the caller ID information. All records are logged to a file in csv format so you can easily parse it for logging into a database, viewing on the Web via CGI, etc.
gnokii is a multisystem tool suite for mobile phones. It provides a library to communicate with a phone hiding the communication protocol. The library handles SMS, phonebook, calendar, phone calls, and other mobile phone capabilities. It supports Nokia-FBUS mobiles, AT-capable phones (most of the mobiles), as well as Symbian-based phones.
PreViking is a telephony daemon written in C. It has been designed with a modular structure that easily allows the addition of different services and the support of different hardware. Current services include a calling card service and a call- back service. PreViking provides an API which includes many functions to do different telephony operations such as call switching, playing/recording messages, and listening/playing DTMF.
SMSLink implements a client/server gateway to the SMS protocol (short messages on the mobile phones). It requires the use of dedicated hardware (a serial GSM module). Both SMS emission and reception are supported, either in text-mode or in PDU-mode. It also includes an SMS to eMail gateway module. The server runs under Linux and FreeBSD and supports interactive mode via telnet. The command-line client already exists for Linux, FreeBSD, Solaris, HP-UX, and Win32. A basic Web interface is also provided.
Linphone is an audio and video Internet phone with GTK+ and console interfaces. It uses the SIP protocol, and is compatible with most SIP clients and gateways. It can use various audio and video codecs such as Speex, GSM, G711, G722, ilbc, amr, Theora, H263-1998, MPEG4, H264, VP8, and snow.
fonmon is a remote phone monitor for isdn4linux. It includes a server application for your ISDN-enabled computer (e.g. an ISDN gateway) and a client for your workstations (Linux and even Windows). You can easily monitor all incoming calls through your network with various clients, with features like highlighting, logging, and number/name exchange. The server won't run on a machine without an ISDN card in it.