I've spent a lot of time in the telephony, call center, crm space and built a lot of applications that involve incoming phone calls and sms. I don't know of a product that works exactly as you specified but I have constructed things that did part of that for conferences and audience feedback. Y...
The viral strategy needs to be a natural extension to the app that requires SMS - not sure you want to just use it for the sake of it being viral (seems forced). A perfect example of this is the Twist app. The app tells someone you are meeting with when you will arrive. When you set up a twist ...