[jacolino]

I must disable the option for simultaneous calls...

[stillabdul]

You can do this in the 3cx Management Console. Go to the extension you want to disable the call waiting for, and then the forwarding rules tab. Under the portion that speaks to Phone busy, there is a setting where you would uncheck "i want to be able to accept more than 1 call at a time". Hope this is what you were looking for.