I used to work in engineering at one of the big wireless telecoms. The impression that I got was that many of the outsourced services were compromised. For instance, we had zero control over our voice mail systems, they were outsourced to Amdocs.

You can see how this benefits the NSA; if the voice mail is outsourced to a foreign company, and the NSA buys intel from that company, it's technically not spying on US citizens, particularly if they're getting metadata.