According to the link below, and several others online, the domain has been suspended (as of 10/23/2011). So, it looks like they are gone! It's probably just a matter of time before all the numbers/callback stop working.
www.callbackworld.com
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile