Below is a listing of open source software having to do with Telephony. For a list of VoIP and business telephone providers please visit Voip-Info.org.
A Java or .Net application to enable the use of a PDA (ipaq) as a bluetooth handsfree set for use in the office, home or car.
(S)MS (I)nitiated (C)alls (U)tilizing (P)BX Systems - Make cheap calls from an SMS-capable cell phone by using Sicup and Asterisk, along with a VoIP provider like NuFone. Text the number you wish to call to an email account to be connected.