A courtesy notification given to the person on the other end of a telephone conversation alerting them to the fact that they are on speakerphone. This is done to prevent unintentionally offending someone within earshot of the speaker. Speakerphone warnings are especially tactful if someone that applies to one or more following traits may overhear the conversation: a minority, obese, a woman, management, your wife, an overly religious person, or a cop.
Antonio: "Hey it's Tony, you're on speakerphone just so you know. My boss, Ernie, is in the office with me."
Bella: "Thanks for the speakerphone warning. Hi Ernie!"