No, I agree that if the "your call may be monitored" is played before every monitored call centre phone call, then Siri should also remind people, at least once every other day. I'm not sure how often people reboot their iPhones, but I don't think doing it just on boot is enough, either.

It would drive the point home if Siri just said, every N queries, "anything you say to me can and will be sent to Apple's servers for analysis purposes".

Call centers don’t know who is calling, and thus add the message every time. It’s essentially always the first message until you identify yourself.

As to regular pop ups or messages, that’s simply a worse user experience for zero benefit.