Getting a phone number is not too difficult for the like minded people. If you are a trader, you must have joined few whatsapp trading signal groups and your phone number is easily visible. Also people can send messages on random numbers hoping to get some victims.
You are probably right, the OP has publicly announced its telephone number somewhere, and that was certainly related to Bitcoin. I use WhatsApp before, but then I just switch completely on Viber, and so far I have had no problem with this type of communication. I am not sure is it possible to block unknown numbers to send you messages or to make a call, but I know it is possible to block any contact (even unknown number) just by open chat with that number and click on block. Such messages can also be reported as spam, so in that way we can help that such numbers become blacklisted, which means that other users will not be able to receive such messages.