I take it back, you are correct. I once had a retail job where I had to pay for my own uniform, so it makes sense to transfer to even tech related things. Good callout!
Even so, asking for a work phone is still doable. But yeah, WhatsApp will still have metadata on you.
True, but s/he must provide you (as their employee) all the tools that are required for you to provide work (of expected quality) at his/her own expense = free of charge for you. Thats the law.
Have you tried refusing to use WhatsApp (and suggesting an alternative)? If so, what was their response?
Expressing your refusal to use WhatsApp let’s them know you’re not happy to use whatsApp, and might make them think about why WhatsApp is bad.
I’d be curious to know what kind of response you get, but I’m not suggesting that you risk jeopardizing your job by pushing back.
If they give you no option but to use WhatsApp, you can try ask them to provide a work device and work SIM card. If they’re nice or if the law in your jurisdiction mandates it, you might end up with a work device you can install all the nasty stuff on 
Based on your previous posts, I guess you just don’t want to install spyware or proprietary stuff on your devices, but is that your only concern? If you’re also worried about WhatsApp collecting your biometrics and behavioral traits, I don’t know what to suggest that would protect you against that. Requiring a phone number to sue WhatsApp sucks too. If you list specific reasons why you don’t want to use WhatsApp, we might be able to suggest how to tackle those.
Curious, what law are you referring to? Because in the US the Fair Labor Standards Act only refers to what tools and equipment an employer must provide. Like if I work in a machine shop I’m not going to bring my own personal drill press to and from work every day.
I’m also wondering since most jobs require their employees to be reachable by phone the company should also pay for landlines at every employee’s house? Should they pay for your preferred method of travel as well?
I don’t know where this sense of entitlement comes from tbh but WhatsApp isn’t the hill I would die on. It’s not like they’re asking you to bring your personal computer, desk, and printer to work.
No, they don’t “need” to. Just like they don’t “need” to pay for your car, phone bill, or even the clothes you wear to work.
I wouldnt do this. In 99% of cases this is the simplest way toi get fired.
Local law that exist (and is in force) in Poland where I grew up.
I guess you are probably right that many workplaces will fire someone for refusing to use whatever they use. Outright refusing is probably a bad idea. It probably wouldn’t hurt to negotiate or ask about it though.
Too late for OP @anon26110603 I guess, but it would be good to find out what tools a workplace uses before accepting an offer to work there.
It’s not useful to say “it’s the law” when we all live in different parts of the world. The laws where OP lives are most relevant in this thread, but OP hasn’t disclosed this.