These clients usually are developed to work with private telephony companies where they use secure access connections through a SIP server to place calls. Being private assumes that you have a valid subscription or a username which is allowed to make calls. Before you are able to place calls, you will need to fulfill these requirements then using their desktop client, start calling using the prefix of the country + the number where you want to dial. You have a virtual keyboard where you can type the number you want to call. Please be aware that these clients use Internet for calling and your Internet connection must run.