Microsoft has launched a new app called Calls+ that allows you to configure call forwarding and call waiting on your Windows Phone. You have the option of enabling call forwarding when you're unreachable, don't answer or when you're busy. Also included is functionality to configure settings for two cellular networks should you be using a dual-SIM handset.
Once installed, you'll need to dive into your device's settings to access Calls+. The app itself offers similar features as Network+, although you do get the option of fine-tuning call forwarding settings (along with being available on non-Lumias).
Thanks Abhishek and Shivam!