I'm trying to integrate with a 3CX system from within our .NET Windows based software application. I have installed and configured the TAPI drivers as per the instructions here https://www.3cx.com/docs/installing-multi-line-tapi/ In control panel, phone and modem section I can see the drivers are installed. As per the instructions in link above, I've ensured the user 1 in the configuration file is the extension (235) configured in the 3CX client. To confirm the configuration is correct, I've tried to use the Windows built-in dialer program to make a call. The dialer can see the different lines I've configured in the config file, as shown below. However, when I try to make a call I receive the error below; A posting on this forum indicates this could be a result of a missing configuration attribute https://www.3cx.com/community/threads/error-with-3cx-tapi-driver-windows-dialer-exe.44948/ I've added in the missing attribute but no joy. Any ideas to what this could be ? I don't think my client is using a Pro license, but will check. Could that make a difference? Thanks for any help.