The first thing to check is the software to see if it has a built-in recording feature, then you can move forward. If Action VoIP lacks this feature, you can take advantage of the already developed applications ready for sound recording.
For that to happen, you also have to enable Stereo Mix. Right click the sound icon on the system tray, and go to Recording. Right click inside the window and select Show Disabled Devices. Right click again Stereo Mix and enable it.
Install Audacity, then select Stereo Mix in the recording menu, place a call and then press on the Record button and you can save the file as WAV, MP3, OGG, etc.
There is also a manual you can check to learn more about various options.