Sky mobile call waiting function

[TraceyB44]

Recently joined sky mobile and call waiting function has stopped working.  Call sky and they said handset function not sim card.  I have s21, call forward is enabled and I've done *43#  phone is dual sim but I've only got one sim in phone so no function to select sim just call forward in supplementary setting.  I've restarted handset twice and nothing.  I've been with O2 for years and always worked only thing that's changed is service provider!