For example, you have to physically go to a store to port the number unless you have the old SIM.
Then it's not done immediately - there's a 72 hour period in which multiple texts and calls are sent to the old SIM asking for confirmation. If you physically have the old SIM this is instant, but if you claim to have lost it you need to wait 72 hours and provide a signature and mugshot at the store.
If a member of staff "forgets" to do this stuff, they go to jail.
People don't usually lose their SIM card, so this process wouldn't happen very often.