Try to stay calm - have a drink nearby to have in case your throat goes dry, try and make sure your background is fairly plain/appropriate etc. It can be nerve-wracking but everyone is a bit nervous, even after your 50th call. It sounds silly, but if you're wearing a jacket/jumper, then choose one that zips or buttons up so if you're too hot or cold then you can adjust without making a lot of movement. Good luck!