They could, but there's also a high chance of discovery. Also, blanket recording hotwords from millions and millions of phones is likely to result in so many false positives as to make the system useless.

For any kind of system to be workable, it's likely they'd have to narrow it down considerably to suspect users. Someone calling the phone number of a person or organization known to be N-degrees separated from real known targets is more likely to yield fruit than scooping up voice conversations from every phone that heard the phrase "bomb making"