Voxpak is a GUI for playing, editing, renaming, etc. voice and fax messages. It includes scripts for popping up sticky-notes or requesters with caller ID info, renaming voice messages to date+callerid, using Python and pyGTK. A small Kaptain script is also available as an alternative.
SCMxx is a console program that allows you to exchange certain types of data with mobile phones made by Siemens. Some of the data types that can be exchanged are logos, ring tones, vCalendars, vCards, phonebook entries, and SMS messages. It works with the following phones: S25, C35i, M35i, S35i, ME45, S45, SL45, M50, and probably some others, too. It basically uses the AT command set published by Siemens (with some other additional resources).
Bluel VMail is a semi-complete voicemail system that supports automated attendant, call transfer, e-mail delivery of voicemail messages, etc. It was designed to work with the Diamond SupraExpress ISA modem and Panasonic KX-T61610 hybrid phone switch (cheap!), but is probably portable to similar configurations.
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.
YaPS (Yet anothewr Paging Software) is primarily designed to send message to pager devices (including cellular phones) which are able to receive SMS. It is a simple command-line driven program with a global and a personal configuration file, but the protocol implementation is kept as a seperate library. YAPS can transfer Pager Messages with UCP, TAP, and Script Protocol.
Ekiga (formely known as GnomeMeeting) is a soft phone, video conferencing, and instant messenger application for use over the Internet. It supports HD sound quality and video up to DVD size and quality. It is interoperable with many other standards compliant software, hardware, and service providers as it uses both of the major telephony standards, SIP and H.323.
SMS spoof is a PalmOS application that allows you to send spoofed SMS messages. It uses a dialup connection to an EMI/UCP-compatible SMSC. It can be used with a modem connected to the Palm, such as an IR link to a GSM phone with a built-in modem. SMS spoof has been tested with Telenor's SMSC in Norway, but it should work with any SMSC that supports the EMI/UCP protocol, as long as no authentication is required.