There are few ways to earn Bitcoin through smart phone and one of the way to participate in signature campaigns. For participating you need to be at least a Junior member and need to post relevant and quality posts to the questions asked in the forum. Go to beginners and help section for more details.

Mining is not possible in smart phone and if any one or any site says you can mine through smart phone, it is 100% not possible since 1 block mining would take 100 years in your smart phone.