Earlier quoted context omitted.
AFAIK you are allowed to use any channel as long as you preserve records of the conversation. However, meeting the standards for preservation is quite difficult, which is why employees of financial institutions are generally told not to communicate outside person-to-person, or channels vetted by their company (and lawyers thereof).
Such a waste of time trying intently to regulate and control communication. People will just get a second private device that is not managed by the organization, and if there is a mutually beneficial advantage to doing so, the other party will do the same as well. This has been going on forever, I remember when they kicked up a huge fuss when they found out that people were doing direct pin-to-pin messages on the bla…
It's not an overall waste of time, because the goal is to reduce the enormous wastes caused by fraud, crime, and other malfeasance.
I know somebody who worked at a bank. The bank had a mandatory vacation policy: you had to be 100% gone for at least two solid weeks every year. When outsiders heard about this, they were often indignant. Who is the company to tell me how I spend my vacation? I know best when I need to rest. Why are they trying to regulate and control so much?
But the policy was about preventing crime. There are kinds of fraud where one person can keep it going a long time if they're around to fiddle things manually. But a couple of weeks of absence, plus the cross-training that goes with it, can keep those kinds of frauds from ever happening. And when they do happen, they stay much smaller.
As an example of why fighting fraud is vital to a bank, you could look at the failure of Barings Bank. One guy was able to fiddle the accounts to hide his losses, gaining a reputation as a trading genius. He started with a little deception, and it spiraled out of control over the years, eventually destroying a bank that had survived more than two centuries.
When compared with the destruction of the bank, making sure that supervisors can see what an employee is getting up to is a pretty small waste in comparison.