The most open-ended way is to navigate to the Dialer tab and type the phone number you'd like to call. Press enter or the call button to initiate the call from your phone.
You can also call your contacts directly from the contacts tab. Simply click on the phone icon next to the number you'd like to call to initiate the call.
Lastly, you may initiate a call from within a conversation. If you are chatting with a single person, you can click the phone button.
If you are chatting in a group message, you can select the specific person to call by clicking on their phone #.