I would like to recommend (if not already planned and in the works) that TING consider bringing some solid, rugged flip- phones to the device line up. A Sprint specific device would be the Kyocera DuraXT.
From personal experience, before I switched from Verizon I had a rugged Casio C781 GzOne Ravine. I could go hiking, canoeing, camping, to the beach, drop it, get it wet, etc. and it never quite on me.
I am now with TING and have an Android phone. I have to baby it. It gets left in the car when I am adventuring. And why not? Who wants to crack, drop or scratch up a $400 phone I use for my work life. But if you had rugged flip phones, I would buy one and add it to my account. Also, I know tons of people that just want a rugged, dependable flip phone and so they stay with Verizon and Sprint, etc. for just that reason. My uncle is one of them. A former Marine that is always fishing, boating, etc. He has no need for Android or iOS, but he does need rugged!
I think a big market is being overlooked.