Acrobits Softphone is capable of running in the background and this is actually the default behavior. You can use the home button or the back button to remove the softphone from the foreground, but it will continue running in the background so you can continue receiving calls. If you truly want the app to close, go to the keypad in the softphone and tap on the menu button. You'll see the option to hide or exit the app. Select Exit to completely close the softphone.
You'll also want to be aware of a couple of preferences that relate to multitasking.
Tap on settings, then preferences.
Start softphone after boot (unchecked by default) - Check this box if you would like the softphone to open in the background every time the android device is powered on. Please note, this will not work if the Softphone is run from the sdcard.
Show notifications (checked by default) - Displays a green icon of a handset when the softphone is running in the background. So a quick glance at your screen will let you know if you are available for calls.