Mine will not turn on. I receive an error message, 'Could not save call waiting setting'. Furthermore, my daughter's phone, iPhone 12 Pro, has the same error. I contact support & they ran me thru the gamete of troubleshooting including a factory reset. This did not resolve the issue so they are escalating it.
Here’s an interesting twist. Turned of Call Waiting in settings and could not toggle it back on. Got the “cannot save setting” message. I also tried the *43# code and that failed as well. Contacted CS through chat and was given the same procedures. The problem was sent up-line.
Received an e-mail the next day with same procedures listed. So, there seems to be no solution.
You might try #43# to try to turn on Call Waiting, but is doesn’t seem to work either.