There is no ‘system’ to purchase as the WP04 phones will talk to each other over an existing intranet. If you would like to talk phone to phone from anywhere in the world to anywhere in the world, you would need to either setup a PBX system (there are many free open source packages available) or sign up with a service that offers voip extension to extension calling. The days of purchasing a massive system and using push-to-talk are long gone, although those legacy systems can still be found. Simply use two WP04 phones, apply static IP addresses to them and they can dial each other over your current network with no internet connection needed. This is perfect, as mentioned in another post, for warehouses, hospitals, hotels, stores, and anywhere confidential but instant communication is needed. Now, if you wanted a warehouse employee in the USA to call a warehouse employee in Australia, you would connect to the internet with the WP04 (or any of the phones sold at Yippz.com) and by dialing an extension, the phone in the USA rings the phone in Australia and your employees are communicating in crystal-clear voip and little to zero cost over the internet — and no salesman sold you a system. Amazing technology.