It just isn't likely. There's no plausible mechanism or reason for an intelligence collection system with keyword analysis to have real-time control of the signalling pathway for the phone system.
Which incidentally works very differently to the way most people seem to imagine phone tapping works -- some agents in trenchcoats in a dark room full of reel-to-reel tape recorders, wearing headphones and carefully putting crocodile clips onto a particular wire, and listening in, hoping the suspect isn't tipped off by crackles or beeps on the wire, or suspicious dropped calls. Maybe in 1950s Hollywood that was how it worked, but now the NSA is just grabbing the content and metadata wholesale from a backbone connection, and analysing at their leisure.
Which incidentally works very differently to the way most people seem to imagine phone tapping works -- some agents in trenchcoats in a dark room full of reel-to-reel tape recorders, wearing headphones and carefully putting crocodile clips onto a particular wire, and listening in, hoping the suspect isn't tipped off by crackles or beeps on the wire, or suspicious dropped calls. Maybe in 1950s Hollywood that was how it worked, but now the NSA is just grabbing the content and metadata wholesale from a backbone connection, and analysing at their leisure.