Microsoft has just released a new app into the Windows Phone Store.
Calls + (also called Calls Extras) is a settings app that will enable you to set your call waiting and call forwarding preferences for your cellular network.
The app supports both single-SIM and Dual-SIM handsets and installs into your Settings list, not app list.
The app appears to replicate most of the functionality of the Calls Settings applet found in the Network+ settings app, so its purpose is not clear, but it does seem somewhat more focussed.
Thanks Abhishek for the tip.