Yes — California's Title 22 §65540 lists an operating telephone as required safety equipment for public pools.
California requires an operating telephone as part of the mandatory safety equipment for public swimming pools.
Citation: Title 22 §65540
Because the phone is classified as safety equipment, it's typically inspected alongside items like rescue poles and first-aid kits. Keep it in working order and test it regularly, since a non-functioning phone can trigger a compliance violation.
SouthEast Telephone installs and monitors cellular emergency pool phones nationwide and can help you meet California's requirements.