Why Do Some Mobile Prefixes in the Philippines Work on Multiple Networks?

I’ve noticed that in the Philippines, some mobile number prefixes seem to work on multiple networks. For example, a number starting with a specific prefix might be originally from one telecom provider but is now active on another. This can make it confusing to determine which network a number belongs to.

Is this due to mobile number portability (MNP) or are there other reasons why prefixes aren’t always tied to a single telecom provider? How do networks in the Philippines handle this, and is there an easy way to check the correct network for a number?"*