Everyone with a telephone should be preparing for a radical change in the way the phone company operates.

You'll still get the usual dial tone on Jan. 1, when the vast AT&T monopoly breaks up: You'll be able to place and receive calls as usual.

But you will no longer have only one phone company, as is the case for most of us right now. Instead, you'll be dealing with at least two companies, and possibly three or more.

You'll have to learn which of the companies to call to get your phone line repaired, straighten out a billing error, change the type of phone-service you're getting or change your telephone equipment.

For local and limited long-distance calls, you will continue to deal with your present local telephone company. But it will no longer be part of Ma Bell. Instead, it will be part of one of seven regional holding companies, each one still a state-regulated monopoly, with rates set by public utility commissions.

Starting Jan. 1, a new charge probably will be added to your local bill: $2 a month for home phones and $6 a month for business phones, for access to out-of-state long-distance service. Some states may also allow an additional charge for access to long-distance calls within the state. This fee must be paid by all phone users, including the customers of non-Bell companies. You'll owe it even if you don't make any long-distance calls.

Legislation now in Congress would delay this access charge for one year, but it doesn't look as if the bill will pass. In future years, your long-distance access charges will go up.

What happens to AT&T, the present company, after the breakup? It becomes a smaller company, specializing in telephone equipment and long-distance service. But in both those areas it will have stiff competition from companies that charge lower prices.

If you are now a Bell long-distance customer, you will automatically become an AT&T customer after Jan. 1. But alternative long-distance companies--among them, MCI, Sprint, Longer Distance, Metrofone and Skyline--will be bidding aggressively for your business. For the first few months, it will remain a bit more awkward to use non-Bell companies. For example, you will normally have to dial 22 to 24 digits to complete a call. But starting in September 1984, the competing long-distance services will receive exactly the same easy access to telephone lines as is now enjoyed by AT&T.

Also starting next September, telephone subscribers will be asked which of the long-distance companies they want to use. If you don't make a choice, your business will be parceled out in some way among all the competing companies in the area. So you might wake up one morning and find yourself a customer of MCI or Sprint.

Last week, AT&T announced a cut in long-distance rates averaging 10.5 percent, starting Jan. 1.

Specifics of the cut--plus other charges that will help determine your long-distance phone rates--will be announced no later than Oct. 3. Toward the end of the year, AT&T's competitors will also be announcing new long-distance rates. Keep an eye on them. If you use long-distance regularly, a non-Bell company is apt to be cheaper.

If you still rent your telephone--paying a fee for it every month--you'll be renting from AT&T starting Jan. 1. But buying your phone will be cheaper in the long run.

Whether you own or rent, you will normally have to take your phone to a service center if it needs repairs--which means that you need a modern, jack telephone that unplugs from the wall. People whose phones are wired directly into the wall will want to convert to the newer jacks. Otherwise, you'll have to pay a hefty fee for house calls each time your telephone has to be fixed. A converter costs just a few dollars at a phone store. If you're handy, you can do the job yourself.

The phone bill you receive in January will reflect this new complexity in the telephone business. It will probably contain several pages: one page for the money you owe the local phone company; one page for AT&T's rental charge for your telephone (if you still rent); one page for long-distance calls (if AT&T is your long-distance company). You can pay all three charges with one check. But by the end of 1984, AT&T may start billing separately. If you sign up with a non-Bell company for long-distance service, it will also bill you separately.

Ready or not, this new day dawns on Jan. 1. It's going to take some getting used to.