For a while I had 2 personal cell phones. 1 was the phone I had under my hubby's plan. He did not believe in texting & would not upgrade the plan. So I went out and bought another pre-pay cell phone & minutes because everyone else wanted to text more than talk. I used it to keep in touch with work, family, and friends. The phone I had under my hubby I kept pretty much just for talking with him through our shared minutes.
After he looked at the cost of both he decided that once our term of service is expired with the contract, we will both be using pre-paid phones. For the time being I am not using the pre-paid phone because our finances are tight.
If you are wondering why a specific person has 2 cell phones, the best thing you can do is ask.