American Telephone and Telegraph Co. announced yesterday the purchase prices and rental fees it plans to charge for its telephone equipment after the divestiture of its operating companies next Jan. 1.

Customers of many of the AT&T operating subsidiaries now have the option of buying their telephones from those companies, but the prices vary. When those subsidiaries are spun off under the divestiture plan, the equipment will be transferred to a new AT&T subsidiary, American Bell Inc., which is proposing to sell or rent it to users at uniform nationwide prices.

In most cases, the guaranteed purchase and rental charges "will represent a decrease" in charges currently paid by AT&T customers, company spokesman Pic Wagner said yesterday. "There will not be many instances of an increase," he said.

The proposed national purchase-rental schedule, which must be approved by the Federal Communications Commission, will have little effect on prices now paid by AT&T customers in the Washington area, company officials said.

The planned maximum American Bell purchase prices for "customer-premise equipment"--telephones already in place--will be: standard rotary, $19.95; standard Touch-Tone, $41.95; Princess rotary, $39.95; Princess Touch-Tone, $49.95; Trimline rotary, $44.95, and Trimline Touch Tone, $54.95.

By comparison, purchase prices for standard rotary phones already in place currently range from a low of $18.75--charged by the AT&T operating company in North Dakota--to $35 in New York, AT&T officials said yesterday.

Bell customers in 34 states can now buy phones they previously rented from local phone companies, as a result of federal regulations that took effect last Jan. 1. Under those regulations, prompted by the AT&T divestiture agreement reached a year earlier, telephone operating companies may abandon their traditional practice of installing phones in customers' homes and charging a monthly rental, or tariff, for their use.

Bell companies in 11 other states and municipalities, including Virginia, Maryland and the District of Columbia, have also asked their regulatory bodies for the right to sell in-place equipment.

The 22 Bell operating companies will be separated completely from AT&T after Jan. 1, 1984. The separation means the operating companies must give to American Bell all of the telephone equipment they had in their inventories at the end of 1982--when the federal government gave them the option to discontinue traditional telephone equipment service.

AT&T yesterday asked the FCC to speed up the process of transferring and "detariffing" the equipment now held by the operating companies, so that American Bell could assume responsibility for the equipment before next Jan. 1. In return for an early transfer, AT&T is promising to continue giving customers the right to rent their equipment, if customers choose to do so.

American Bell's proposed monthly rental fees are: standard rotary, $1.50; standard Touch Tone, $2.85; Princess rotary, $3.15; Princess Touch Tone, $4.05; Trimline rotary, $3.45; and Trimline Touch Tone, $4.60.

The proposed rental fees also would stay in effect for at least two years.

Current monthly rental fees for standard rotary phones range from 90 cents in Maine and several other states to $3.03 in New York, according to AT&T officials. An estimated 42 percent of the current customer premise equipment is standard rotary.

AT&T now rents an estimated 100 million telephones in the United States. That means the company also pays for maintenance of that equipment. Customers buying their phones would have to pay for their own maintenance, in much the same way that they pay for the repair of other personal property, such as television sets.

Since late 1982, AT&T has sold 3 million of its in-place phones, company officials said.

AT&T plans to spend $12 million over the next year in an advertising campaign advising customers of their right to buy in-place equipment.

The company also said that it is offering a 30-day warranty on all purchased telephones already in use. It will offer a 90-day warranty on purchases of new telephones.

AT&T officials said yesterday that their purchase prices are competitive with those of other companies, such as Tandy Corp., owner of the Radio Shack chain; Sears & Roebuck; and W. Bell & Co. American Bell has 13 stores in the Washington metropolitan area.