From VDR Wiki
This plugin is a very simple (read: not very featureful and hopefully with an high WAF ;-) softphone using the iax protocol (asterisk server).
It uses the iaxclient library to handle every details of the call, so the plugin is just an user interface.
- A free (i.e that isn't used by something else, like a softdevice plugin), full-duplex sound card.
- development version of vdr
See plugin installation.