According to Reuters, the Justice Department is specifically interested in whether or not the merger will affect prices of MVNOs that purchase network access from T-Mobile and Sprint.

It's noted that:

"The Justice Department, which is evaluating T-Mobile's $26 billion deal to buy Sprint, has been speaking with small wireless operators that buy access to the major wireless networks at wholesale rates, and is seeking their opinions about the merger, the people said, who declined to be named because the talks are confidential."  

How are these smaller MVNOs responding? So far, it's a mixed bag.