If you don't wear a helmet, a lower cost solution is a Zulu headset and plugging in an Iridium portable sat phone. (The Zulu will interface a cell phone via Bluetooth and the Iridium sat phone by a patch cord.) Iridium just released a smaller unit in December. Here is a picture showing the new smaller unit versus the previous model.
Another option is a PS Engineering 8000B audio panel -- it has a port to plug in a cell or sat phone.