Why are wireless prices decreasing?

This article from the WSJ reports that wireless "carriers are waging a price war to attract new customers" and that "churn", the movement of a customer from one carrier to another, has increased. 

SUMMARY: AT&T, Verizon say they need to work harder to expand subscriber counts; T-Mobile unveils a new deal.

CLASSROOM APPLICATION: The article prompts the question about the factors that are causing lower prices among wireless providers. While the article reports on the competition to attract new customers, it does not examine the possible causes. Two factors about industry structure that affect equilibrium prices are quality differences among wireless providers and costs to consumers of switching providers. Students can discuss whether shrinking quality differences among providers and reductions in switching costs leads to lower equilibrium prices.
QUESTIONS: 
1. (Advanced) What is the effect on a price war among wireless carriers on whether consumers switch carriers? What is the "prisoner's dilemma"? In what sense is a price war among wireless carriers like a prisoner's dilemma?

2. (Advanced) Research question. What factors are causing wireless carriers to engage in a price war?

3. (Introductory) Research question. Why is T-Mobile gaining customers?

Reviewed By: James Dearden, Lehigh University