You can dial a phone number from the Dialer window (see picture below). To display this window go to the Tools menu and select the Dialer option or click the Dialer button on the toolbar or press CTRL+D.

You can type in the number that you want to dial or select it from the last 10 dialed numbers. After that you have to click the Dial button. Click the Redial button to redial the last dialed number.

To erase the number that you typed click on the C button. To erase the last typed digit press Backspace key or click on << Button.

You can associate phone numbers with names and store them in the memory, same as a regular phone does. You have to click the memory that you want to use then type the name and the phone number. When you will close the Dialer this information will be saved automatically.

Click on Close button or press the ESC key to close this window.

During dialing, the Dial Status window is displayed.

Notes
The comma (,) is a standard 2 seconds pause between digits.
You can enter the phone number using your numeric keypad as well or clicking the 10 digit buttons.
Be sure to specify the proper COM port and other modem options in preferences.