I've done an interview by telephone before (not for NATS or ATC). Not sure if your question was specific or more general. If former then please ignore remainder
Usually used when the distance need to travel means that they want to be more sure before inviting you to a face to face.
They are tricky because you cannot get any feedback from body language (have you spoken long enough, too long, are you heading down wrong track etc).
Main thing is make sure you get the right environment for your end of interview: quiet, no interruptions, drink to refresh yourself, notepad if you want it. Have your CV etc in front of you so that you do not contradict it. have info on the intended company as wel in case you need to check
Have a few questions you want to ask written down on your pad
Because you cannot read body language, make sure that you understand the question fully. Ask "Did you mean" type questions
All IMHO naturally (and yes I did get offered the job when I had one)