Interviewers assess competences the same way as they would face-to-face - by asking questions around examples of work-related events. For example, tell me about a time when you felt under pressure at work. Then a series of follow-up questions around how you dealt with it.
Do you know what core competences are regarded as essential for the work? Will they tell you? (they might not).
Prepare by writing up in notes examples of common traits required in most jobs - writing notes helps you by fluent when the time comes to draw on your experience to give an answer.
One of the (few) good things about a telephone interview is that the other person can't see what crib notes you have in front of you.
If the organisation is worth anything, it won't attempt a complex and deep interview over the phone - it doesn't help either party. It is best used to sort the wheat from the chaff - the real interview comes face-to-face.
It is more often usefully used to test out the worth of what is on the CV - not to test competences.