Are you using the makecall sample and giving one number as external number?
This does obviously not work for external numbers because the sample makes a call, answers it and the hangs up. How would jtapi have an external call answered? It cannot because it has no control over that number, and hence the error message which tells you that jtapi cannot control that (external) number.
But you can rip out the code to just make a call and use it in your own software and that will work just fine.