American Telephone & Telegraph Co.'s net earnings rose by 21.6 per cent over the previous year and restored "a trend interrupted by the recession," chairman John D. deButts said yesterday.

AT&T, the nation's largest utility, said its 1976 earnings were $3.829 billion ($6.05 a share) compared with $3.147 billion ($5.13) in 1975. Operating revenues totaled $32.816 billion compared with $28.957 billion.

But deButts said the 1976 earnings represented only an 8.9 Per cent return on total capital, less than the 9.5 per cent the Federal Communications Commission has authorized for the Bell System's interstate services.

AT&T said operating expenses for 1976 rose 12.1 per cent to $21 billion from $18.8 billion in 1975.

DeButts said teh company's improved earnings were aided by "rigorous management of resources, stronger growth in demand for our* services growth in demand for our services, an active marketing program, continuing advanced in technology and im provements in operation methods."

The chairman said it was essential that earnings contiune to improve to attract the "continuing investment in new facities on which our ability to provide good service depends."

AT&T sad prevously it would spend some $11.4 billion this year to improve telephone service and handle anticipated growth in demand for service.

At the end of 1976, the Bell System had 123.1 million telephones in service compared with 118.5 million a year earlier, a 3.9 million increse.

Total long-distance messages rose by 7.9 per cent to 11.7 billion from 10.8 billion.

In 1976, AT&T became the first American company to earn more than $1 billion in a quarter, reporting net earnings for the three-month period ending Aug. 31 of $1.01 billion.

The telecommunications giant's fiscal year coincides with the calendar year, but for purposes of dividend payments the company makes quarterly reports off the calendar year by a month - on a Dec. 1-Nov. 30 basis. In its fourth quarter ended Nov. 30, AT&T reported net income of $1.03 billion ($1.61) compared with $805.7 million ($1.30) in 1975. Operating revenues for the period rose to $8.432 billion from $7.503 billion in the same period a year ago.

The company's calendar year earnings included results from its Long Lines Department, the Bell Telephone companies and the Western Electric Co.

AT&T said last year it paid out $15 billion in wages and salaries to some 927,000 employees, more than $4 billion in federal, state and local taxes, and nearly 15 billion in dividends and interest to its 5 million investors, the largest member for any American company.

Two top steel producers say 1976 showed moderate economic gains after the industry-wide slump of 1975. They agreed that last year came in with fiscal sparkle, but cost pressures and a weak capital goods market forced it to leave with sputter.

"After a strong showing in the first three quarters, shipments fell of in October and November" said George, A. Stimson, chairman of third-ranked National Steel Corp. "This reflected a developing slack in the economy and resultant inventory corrections by our custoers."

National's earnings increased only 5 per cent in the fourth quater of 1976 but were up 48 per cent for the year when net income reached $85.7 million ($4.53 a share) compared with $58 million ($3.10) in 1975. Sales in 1976 were $2.8 billion, up 27 per cent from the 1975 level of $2.2 billion. Stinson said the cold weather and fuel supply problems also affected fourth-quarter developments.

Allegheny-Ludlum Industries reported sales were up last year but earnings were down slightly.

Net income in 1976 was $30.7 million ($3.41 a share) on sales of $889 million. This compares with earnings of $31.8 million ($3.59) on sales of $722 million in 1975, the 11th-ranked steel producer said.

"A record surge of speciality steel imports, principally from foreign producers who are subsidized by their governments. . . contributed importantly to a market environment that prevented us from realizing prices . . . adequate to offset higher costs," said Robert J. Buckley, Allegheny-Ludlum president.