If you are starting with graham cracker sheets, pulverize them first in a food processor. Skip and go to the next step if starting with crumbs.
In a small bowl, stir together graham cracker crumbs, sugar, and melted butter until the crumbs are moistened.
Pour mixture into a pie plate and use fingers or side of measuring cup to firmly pack crumbs onto the bottom and up sides of the pie plate. You'll want to be firm enough that the crumbs stick to the pie plate, but not so firm that you'll have a hard time cutting through the crust.
Place in fridge for at least 30 minutes to firm up.
Place cream cheese and sweetened condensed milk into bowl of a stand mixer.
Starting on low speed and working up to medium speed, whisk together sweetened condensed milk and cream cheese until well combined.
Add calamansi juice. Starting on low speed and working up to medium-high speed, whip the mixture until thickened and well combined.
Pour the filling over the crust and spread it evenly. Refrigerate overnight to firm up. You'll be tempted to try to cut it before then, but this filling is very soft and really does need the time to firm up enough to slice through.
Before serving, garnish with calamansi zest and graham crumbs if desired. Squeeze some fresh calamansi juice over the top of your pie if you like an extra punch of tartness!