American Telephone & Telegraph Co., in what a spokesman says is a preventive move, has gone to court here to make sure that a U.S. bank does not pay off a possible $30.2 million Iranian claim.

In what could be the first of a jumble of financial fallouts from the Iranian revolution, AT&T late last week asked the court to enjoin Manufacturers Hanover Trust from complying with any Iranian attempts to collect on a letter of credit designed to ensure AT&T perform properly under a contract to the Iranian government.

The suit itself, like much of international finance, is complicated. But, an AT&T spokesman said, the issue is simple; AT&T has not been paid for work it did on engineering and designing a communications system for Iran.

AT&T's American Bell Interntional Inc., like nearly all U.S. companies doing business there, has since pulled out of the country.

A spokesman for AT&T said the Iranian bank has not made a claim against Manufacturers Hanover as far as he knew, but that the company wanted to make sure it was prepared in case such a claim was made.

A spokesman from Manufacturers Hanover said it would have no comment on the AT&T suit.

AT&T has been working on engineering and design of a national communications network for Iran for three years, the company said. The contract was renewed last July for about $194 million, to be paid in monthly installments on the basis of vouchers the AT&T subsidiary presented to the Iranian telecommunications industry.

That contract called for Iran to make a down payment of $38.8 million to the AT&T subsidiary. As the work was performed, the size of the down payment was to be reduced gradually. To ensure that AT&T did the work properly, the Iranian government could recall whatever amount AT&T had not billed against the down payment. That is down now to about $30.2 million.

AT&T, for its part, secured from Manufacturers Hanover a letter of credit, the size of which declined as the size of the down payment was reduced by periodic payouts to AT&T.

If the Iranian government went to Bank Iranshahr to demand a refund of the remainder of its down payment, the Iranian bank could then collect that amount from Manufacturers Hanover. Whatever Manufacturers Handover paid out then would become a loan at AT&T.

The AT&T spokesman said that the project its subsidiary was working on already had been scaled back by the monarchy before it toppled.

"At the height" of the project, he said, AT&T had 900 persons at work. About 400 were working on the national communications when the antishah disturbances intensified last fall.

AT&T, at the imperial government's request, had developed a proposed project that would have employed between 100 and 200 Bell employes. Although Bell had pulled all its employes out of Iran, the spokesman said if the new government wants AT&T back, the phone company would be willing to talk about it.

AT&T, in its suit seeking a temporary order stopping Manufacturers Hanover from honoring any Iranian claims, said it is owed about $3 million for work it did in Iran in November and December and part of January for which it has not yet been paid.