How is Telos different from PTT service from cellular carriers?

PTT service from a cellular carrier such as Sprint Direct Connect is a very useful feature. However, you have to use a dedicated cell phone from that carrier and you have to be a paid subscriber.

Telos does away with all those restrictions and turns your smartphone into a PTT-enabled-device that can be used with any carrier. On top of that, Telos PTT is completely free of charge.