WhatsApp is a masterclass in network effects. You can no longer decide whether or not you want to use it. Because your employer uses it, you have no choice but to use it. The only thing that will disrupt this is if security concerns make companies come out and explicitly ask employees not to use WhatsApp and I don't see that happening any time soon.
If your employer insists on installing and using a specific application on a phone, ask them for a phone to use it on. Don't feel the need to install it onto your personal device.
