AT&T Corp., as expected, announced it has begun offering a next generation wireless service dubbed AT&T Digital PCS in 40 markets. The service, launched by Seattle-based AT&T Wireless--formerly McCaw Cellular--offers paging, personal messaging and phone service in one product. The service costs $25 per month and 60 cents per minute. A campaign by McCann-Erickson Worldwide, San Francisco, will promote the new service. Sprint Spectrum, an alliance of Sprint, Tele-Communications Inc., Cox Communications and Comcast Corp., is readying a nationwide launch of a similar product.