When I use the 3CX-phone in CTI-mode with a Yealink T42, I cannot select a new line to make a new call when I already have a call. There are no linekeys as in the Softphone-mode.
Please note that you can add line keys from the management console. Just edit the extension navigate to the BLF tab and add line keys. Reprovision the phone to apply the keys
You can also place the person you are talking to on hold and click on new line button. This will allow you to place another call. You can switch between calls using the arrow keys
When the client is in CTI you need to use the IP phone to open a new line as the call is established on the IP phone. This is the way it is designed to work.