Other methods of gaining coins

While I'm completely aware that coins make the developer money, surely there is a way for us to gain coins in-game that isn't inviting someone?

Perhaps

• 1 coin for completing x amount of missions (1 coin for every 1000 calls dealt with?)
• 1 coin for playing the game for x amount of days in a row (1 coin for playing the game for 1 month / 28 days straight?)

Just some ideas.