Scott: Though I've researched this a fair amount, I'm certainly not a lawyer. My understanding was that if you notify and the other party continues with the call, they are consenting. Looking a little closer, this seems to vary by state. I've updated the line in question to include the clause "and obtain their consent". Thanks!

Ultimately, as originally suggested, if there's a legal concern a real lawyer should be consulted. My goal was to make folks realize that this is not some nefarious activity - it can generally be done, particularly with consent, for all sorts of useful archiving.