But, here is a little help - jailbreaking. Only you need is 'Backgrounder' app that you can find in Cydia. (also you may want to install 'VoIPover3G' or 'Unrestrictor (paid-app)' so that you can make a truphone call over 3G)
One important thing I recommend is to turn on 'Vibrate' of Truphone setting in 'Settings > Truphone' If you miss doing this, you may not be awared that your truphone app is receiving a call!
Once eveything's ready, you'll be able to receive a truphone call with your truphone app running background! (You need to understand that truphone is receiving a call if your iPhone is vibrating with no indication of any pup-ups like the built-in phone app does)
Here's a movie shot: