Good. We're moving on to a different argument, and abandoning the "piss" metaphor. Excellent.
You cited this law. Specifically, 632(a).
I would direct your attention to the definition of "confidential conversation", at 632(c):
For the purposes of this section, “confidential communication” means any communication carried on in circumstances as may reasonably indicate that any party to the communication desires it to be confined to the parties thereto, but excludes a communication made in a public gathering or in any legislative, judicial, executive, or administrative proceeding open to the public, or in any other circumstance in which the parties to the communication may reasonably expect that the communication may be overheard or recorded.
Upon any individual on the call stating "This call may be recorded", every party present "may reasonably expect that the communication may be overheard or recorded". When that happens, the call no longer qualifies as a "confidential conversation". As it is no longer a "confidential conversation", 632(a) no longer applies. As it is no longer a "confidential conversation", any party to it may record it.