No, most schools do not start calling people as soon as spots open. Most schools wait until the April 15th deadline and they have received an answer from everyone. Which typically means until the end of the business day on April 15th for people to accept an offer or not. Then, after that period, they review alternates and start calling to offer people spots for open spaces. Those people are given some time to make a decision and then another batch of people are called if there are still open spots. That continues until all spots are filled. While some schools may call sooner than April 15th, if you are alternate your best bet is to not expect anything before April 15th and maybe not even until April 16th all the way through until classes start. And even there are some rare but random instances where people are called a week into classes starting to be offered a spot. Not often, but it does happen.