Before the installation of DIAL|IT you have to install a tapi driver or skype. If both are installed you can choose how you want to use DIAL|IT. But if you haven’t installed either of them, it doesn’t make sense to install it.
Why using a tapi driver?
Tapi is an application programming interface for phone applications. You need a tapi driver, so that you can use a tapi compliant phone.
The tapi driver, which you become from your phone provider, will be installed of your computer. It allows the access to the phone functions.
Check in the following link with phone.exe if your telephon system is tapi-conform.
Why using Skype?
Skype is a VoIP-Software with Instant Messaging-Function, file transmission and telephone functions.