As of iOS10, this article has been edited.
Previously, there was an additional VoIP method for backgrounding, but now Apple has deprecated the feature. Now the only options for receiving calls when the app is not open and in the foreground are Push Notifications and Standard (formerly referred to as Keep Device Awake).
Unless you have some provider who has restrictions about where you can register from or you have a PBX only available on a local network, Push will be the way to go. It is far more battery efficient and with the recent addition of CallKIT, the experience of using them is almost identical to the experience of receiving call on the native iOS phone app. The Standard method works, but will be more demanding on the battery.
Go to the article below to configure Push Notifications on Acrobits Softphone and Groundwire.
Go to the article below to configure Multitasking on Acrobits Softphone and Groundwire.