Voip is a project mainly written in PHP, it's free.
Basic modules of the Voip Drupal framework
-- SUMMARY -- The VoIP Drupal module enables Drupal installations to interoperate with VoIP providers and virtual PBX installations like Twilio, Asterisk, and TrixBox.
For more details, visit the module's project page: http://drupal.org/project/voip_drupal
To submit bug reports and feature suggestions, or to track changes: http://drupal.org/project/issues/voip_drupal
-- REQUIREMENTS -- Access to a VoIP provider supported by the module. At present, this is Twilio only, but we plan to support Asterisk and others in the very near future.
-- INSTALLATION -- Install as usual.
-- CONFIGURATION --