I would always advise you to prepare. If it is a quick phone call then no harm done, but if they want to ask you questions then fair enough. Recruiters often call to filter people down, to ensure that the person is not wholly inappropriate for the role. I would suggest you prepare to be asked why you are looking for the position, the soonest you would be available to start, and the skill sets you have you feel will match the position advertised.
In addition I would write down the questions which are most appropriate to you about the job. Not a whole flood as that would be more appropriate at the face to face interview. However, it will give an indication that you are intelligent and articulate. Have a pad by the phone. Maybe come back to some of the points they have made earlier about the role. And just remember they probably have in front of them what you have written on the application form.
It is sometimes easier on the phone, as they cannot see any outward signs of nervousness. And if you get to the face to face stage, once you get past the initial butterflies you will be fine.
Good luck.