Bring water and sugar to a boil, add zest, turn off the heat and allow to steep for 30 minutes. Hydrate the gelatin leaves by soaking them in cold water. Warm up the steeped syrup. It should not be simmering or boiling. Squeeze the excess water out of the gelatin and add to the syrup. Stir until dissolved. Add the calamansi juice and stir to combine. Pour the mixture into a bowl through a fine-mesh sieve to take out the zest. Allow to set overnight. Once set, put jelly into a stand mixer with the whisk attachment. Whisk for 40 minutes on medium-high speed until really frothy. Scrape down the sides periodically. It should look fluffy like a meringue but will not hold peaks. Meanwhile, seal one end of a ring mould with plastic wrap and place on a flat tray. This will make sure the mixture is contained within the mould and is easy to unmould later. Once the whipped jelly is white and frothy, pour it into the prepared ring mould and place into freezer immediately. Freeze overnight. Once set, remove the plastic wrap and run a knife along the inside of the ring to demould. It should slide right out. Cut into desired shapes and serve immediately. It is best eaten cold. If left at room temperature for too long, the mixture will become soft and lose its shape.