First, make sure that your security program doesn't block the connection. After that, go to Start > Control Panel > System and Security > Windows Firewall > Allow a program through Windows Firewall and check the box next to SIPLink. If you can't find the application in the Allowed programs and features window, press the Allow another program button, navigate to the installation directory, select the executable file and press Add. After that, from the Advanced Settings select Inbound Rules > New Rule. Check the Port box, click Next, select All local Ports and follow the self-explained steps. If you still have issues, try uninstalling the application and then installing it again. Also, it is possible that the SIPLink servers might be temporarily down or under maintenance. As an alternative, you can use another VOIP utility from Software Informer database.
