Your scenario is very possible and was my initial thought as well. The call dropping could be carrier/phone/region specific due to invalid or error causing commands breaking something when the call logging or recording is activated. (tower connection, phone software, phone radio firmware, or anything else along the connection)

It's probably been fixed by now if it was a bug such as that. I'm sure bugs like this go unnoticed every day by people interacting with computer systems and just brush it off as a "glitch" or "service issue".

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.

If a system can parse for keywords, any tracking or recording is already enabled.

It's also not how the phone system works.