An audible beep tone is legally sufficient, I believe. You hear it when talking to some banks or stockbrokers. Advantage of this approach is that it covers your butt, but hopefully the customer service droid won’t be hip to what it means.
1500hz every 15 sec.
Interesting other question: does a “this call may be recorded…” message at their end imply consent to recording of the call by you as well as them?
