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).
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.
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.
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.