It's just on work phones, they can do what they like on their personal phones of course. However, if they are discussing anything business related they'll be fired and could be barred from working in the industry altogether.
HSBC etc completely lock down Teams etc, there is no private anything, it's all monitored as long as its on their devices and accounts.
Traders and other front office staff will have every single phonecall, email, IM, voice chat, and video call monitored and stored for something like 7yrs.
People are completely missing the point here.
The banks are required to record all of the communications made by a large proportion of their staff. Probably they haven't found a way to do that for WhatsApp or text, so those are not approved communications channels. If they aren't approved, then why would they be on a work phone?
Of course the staff can break whatever rules they want, but they will be breaking rules. The banks make sure their staff know that, and as long as they've done all they can to stop it then what else can they do? There are always people who break the rules, and those who break the rules get punished when they're caught.