Windows Phone Operating System has historically not allowed VoIP phones to run in the background. This is needed to allow your phone to be "active" and receive calls; when your busy using other applications on your phone.
However, in Windows Phone 8.1 (Released July 2014), Microsoft improved this and provided background support. As of writing (Nearly a year after 8.1 was released) we are yet to see any app developers properly implement and support background calling. So therefore as with Windows Phone 8.0 (The prior release), to receive calls you need to have the app in the foreground as your current application. The two major soft phone apps on Windows are called Zoiper and Linphone. Zoiper claim to support background calling if their service provider sends an additional "X-PUSH-URI" in their SIP packets and "Push notifications" are turned in the Zoiper app. We've tested this extensively with Zoiper and it just doesn't work in their current release!
So for now, if you're running a Windows Phone, then unfortunately the world of VoIP isn't the smoothest experience for you. No problem for making outbound calls as by virtue you will open up the application and dial, but for receiving inbound calls, you'll need your Zoiper or Linphone constantly sitting and waiting for the call in the foreground. We will update and/or remove this FAQ when we see a Windows Phone app that does provide stronger support for the latest functionality released in Windows Phone 8.1.