I'm so fucking done with advertiser's messaging me on WhatsApp
No really. What do they think? People are gonna have any faith in their product or service after they are annoyed reading their shitty bulk message?
These days in particular it's been happening a lot lately, a friend of mine suggested I google my number nunber. Lo and behold I saw several entries. I then looked up my family members numbers as well and they were all right fucking there. Somehow I guess some database out there gets leaked eventually and then these firms (probably from some third world country) end up crawling through leaks and sell to the highest bidder.
No matter how many of these accounts I report for spam, more keep coming. I'm genuinely considering getting a new phone number but then again there's no guarantee how long that'll last. Seriously, fuck these guys and their dumpster fire ads.

Kinda why I moved to iMessage
It sounds like someone installed an app on their phone and granted it permission to access
Contacts. Of course it wasn't you, but all it takes is one weak link that leads to hundreds of contacts being leaked out.I'm afraid that we might need a spam folder to filter out these advertisers.
Considering a lot of apps on your phone have access by default: messages, phone, etc. It could have well been one of them.
Not sure about apple, but on Android one can deny Contacts, messages and phone permissions.