When I was covering the Consumer Electronics Show in Vegas (I write for iPhone Life Magazine -
http://www.SmartphoneMag.com/iPhone/Todd.Bernhard) there was a vendor (Newber,
http://www.MyNewber.com) showing a call manager iPhone app that was pretty powerful. They were having trouble getting it approved...their website even has a petition and a timer showing 114+ days waiting for approval. The app allowed you to transfer a call to a landline phone without the end user noticing. It uses GPS to remember what landline phones are nearby! You get a Newber number and give that to contacts. You can always accept calls with your iPhone's native number, but it allows you to send Newber calls direct to voicemail.