I recently purchased an iPhone 4s from a man I know. I put in the SIM card and then turned it on?

I recently purchased an iPhone 4s from a man I know. I put in the SIM card and then turned it on?

I then decided a week later to go ahead and reset it. I resettled it and now when I set it up I go to the settings and it says my number is unknown. How to fix this?!