There is no need to install additional apps for calls. On CRM desktop screen, in the bottom right corner click on a green phone button.
Enter a number manually, choose from contacts list or call your colleagues.
You can always see call history in the appropriate tab on the left.
You can also accept incoming calls via a web-browser. If a browser is not always convenient, you can install programs, apps for iOS/Android or set up call forwarding to your number in PBX.