You do need to know the location of both parties.
U.S. Federal law (18 USC 2511(2)(d)), which prohibits the interception of wire and electronic communication, states:
It shall not be unlawful under this chapter for a person not acting under color of law to intercept a wire, oral, or electronic communication where such person is a party to the communication or where one of the parties to the communication has given prior consent to such interception unless such communication is intercepted for the purpose of committing any criminal or tortious act in violation of the Constitution or laws of the United States or of any State.
California Penal Code 632 requires the consent of all parties to a confidential communication in order for the conversation to be recorded. The statute defines a "confidential communication" as follows:
The term “confidential communication” includes 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.
Once AT&T, or anyone else for that matter, states that the communication is being recorded, it is no longer considered a confidential communication requiring the consent of all parties in order for any party to record it.
Part of the California Civil Jury Instructions for this code requires that the plaintiff must prove, among other things, that the plaintiff had a "reasonable expectation that the conversation was not being overheard or recorded."
You can see the full jury instructions here: https://www.justia.com/trials-litigation/docs/caci/1800/1809.html