Do your research and look into the courses you are calling about. You're making a pretty big decision, as you will be living and breathing this course for the next 3 years, so don't go into it blind.
Do ask to speak directly to teaching staff if possible – that way they can give you a real flavour for the course. Have some constructive questions in mind and make it clear that you have put some thought into whether or not the course is right for you.
Do have a note pad next to you, so you can jot down any notes during the call. Have your details to hand – for example your UCAS number, Clearing number and your A-level/GCSE grades. It may be handy to have your CV to hand too in case you are asked about your work experience etc.
Don't panic - thousands of students will be picking up the phone like you and considering their options. If you didn't get into your number one choice, this isn't the end of the world. I am a big believer that things happen for a reason.
Don't hang around too long - start calling as soon as you can, as places can get filled up quickly. But at the same time don't rush into accepting a place on a course before you've thought it through. Don't assume anything, if you are uncertain about something, ask.