Why bother with employees - just go give money to intel to do this. Intel as a system is designed to produce chips that work a certain way, and my understanding is that said system is rather good at what it does, dedicating the time and energy of many rather smart people to making sure things work the way they're supposed to. Why risk throwing a monkey wrench into such a system when you can just point it in a different direction?
AT&T and Verizon don't have 'plant' employees. Much simpler - and legally, safer - to just give the bag of money straight to the corporation.
There's more than one country interested in pulling off these attacks. The US can say, "Do us this favor and we won't look too hard at your tax avoidance schemes," but China or Russia might have an easier time planting a few employees.
When you have the kind of money and influence that the NSA and CIA have, you probably do both.
As for other friendly and less-friendly nations, I'd be very surprised to learn that there weren't any other nations represented in the ranks of Intel engineers.
AT&T and Verizon don't have 'plant' employees. Much simpler - and legally, safer - to just give the bag of money straight to the corporation.