There are no such thing as burner phones in many countries any more. You have to show ID to buy a SIM card, and a copy is made and sent on to the authorities.

Also, Signal revalidates the number when restoring from backup. If you don't have the number any more, you can't revalidate and have to start over with a different number.

I bet you 50 euros that I could pay a random homeless person or drug addict to go buy a basic android phone with SIM card and service at retail. The sort of people who are running drug smuggling and middle/end-retail distribution operations have access to lots of such persons.

'smurfing' methodologies are well known.

They can steal a SIM, or buy one from a country where they are available freely.

Belgium is one of these countries.

I'm not going through the hassle of buying a new phone just to create another centralized chat account. I have email (with GPG if you don't mind that,) a jabber account with OMEMO enabled clients, and SMS. Two people have bothered to ask if I want to use signal and everyone just uses SMS.

Are you a member of an organized crime syndicate?

Oh I totally agree, I think the phone-number requirement is absolutely ridiculous. Just pointing out that it's possible.