Begin by washing the red chilies thoroughly. Remove the stems and seeds, then slice them thinly. The seeds can add more heat, so adjust according to your preference.
In a dry skillet over medium heat, toast the belacan for about 1-2 minutes until it becomes fragrant.
Using a mortar and pestle or a food processor, combine the sliced red chilies, toasted belacan, salt, and sugar. Grind the mixture until it reaches your desired consistency.
Once the mixture is well combined, stir in the calamansi lime juice.
Taste your sambal and adjust the seasoning if necessary.
Transfer the Sambal Belacan to a clean jar or container. It can be used immediately but tastes even better after a day in the fridge.