Use the Windows telephony device
When this option is enabled, Custom Addressbook uses the built-in Windows Telephony API service when you double click on a field of type 'phone' or 'fax'. Please note that the proper hardware and drivers for this need to be present on your system.
For instance, this can be used when dialing out via a modem which is on the same line as a phone.
Use a VOIP program (such as Skype)
When this option is enabled, double clicking on a 'phone' or 'fax' field activates a 'voice over IP' program (such as Skype) that connects over the internet. Please note that a proper program for this needs to be installed on your system.