Here's how Jan. 1 breakup of AT&T will affect telephone bills and service

What will consumers have to do when the great American Telephone & Telegraph monolith divests itself of its 22 operating companies on Jan. 1? ''Nothing - nothing at all,'' says AT&T spokesman John Geoghegan.

If you pick up your phone to wish your friends a happy new year, the telephone service should be the same whether you place your call before or after the glowing ball makes its descent from the Allied Chemical Tower into Times Square.

The first sign that something has happened should come in your January phone bill, which will probably be a three-in-one affair: one bill for local calls from the local phone company; one bill for long distance from AT&T Communications; and, unless you already own your telephones, a bill from AT&T Information Systems for equipment rental. Billing for other long-distance carriers, such as MCI, will continue as before.

The hotly debated long-distance ''access charge'' will not appear until April 3, if then. Local service rates are expected to rise, but the rate-increase mechanism will proceed independently of the divestiture date. New charges for interstate directory assistance are also in the regulatory pipeline, but will likewise be independent of the divestiture date.

If you need repair service after Jan. 1, the first thing you need to do is to determine whether the problem is in the telephone instrument or in the line. If your phone works when plugged into your neighbor's jack, the problem is on the line, and you should call your local phone company for repair service.

If the problem is the phone itself - and the phone is an AT&T instrument - you will be able to mail it to AT&T for repair, drop it off at one of 900 Phone Centers across the country, or - for a rather steep price - have an AT&T technician in for a house call. If you bought another brand of phone, repairs are up to you.

Around May 1984, the local phone companies will start polling customers as to which of the long-distance carriers they prefer - AT&T, MCI, Sprint, or another. Alternative long-distance carriers will be phased in on an equal-access basis (with no 12-digit numbers to be punched in first) in September. Once a customer has designated a specific carrier, all his calls will be routed over it. But for some rural customers, AT&T will be the only long-distance service.

Many details have yet to be worked out: whether local phone companies will handle billing for long-distance carriers other than AT&T, for example, and what will happen to customers who do not designate any particular long-distance carrier. For those with questions, AT&T is making available a toll-free number: 800-555-5000.