Source: Google Images
In this town, there are lots of vendors who sells "Kalan-banga" (Earthen jar)
One day, a Spanish soldier came and it seemed unaware of his current location. He asked the people him around about the name of their place.
A nearby vendor overheard the Spanish soldier but the vendor did not understand what the Spanish is asking.
Thus, the vendor mistakenly answered "Ah kalan-banga?"
Later on, Kalan-banga was shortened to "Calamba".