Bring water to a boil in a medium saucepan. Place tea in a tea sock, add to the boiling water, remove from the heat, and let steep for 5 to 10 minutes, depending on how strong you like your tea. Strain the tea into a pitcher and stir in the condensed milk. Fill 4 highball glasses with crushed ice, pour in the tea, and stir to chill. Add more sweetened condensed milk, if desired.