Activision has detailed how it will use an SMS verification system as a requirement to play Call of Duty: Modern Warfare.According to the company, players must own a text-enabled mobile phone number to play the game - but only on PC. Earlier this week it was reported that Modern Warfare 2 would require players to register a phone number to play the game. It was also noted a pre-paid or VOIP number might not work with the system. At the time, it wasn't specified that the requirement was only for PC players, leaving many players concerned. Read more