American Telephone & Telegraph Co. and the striking Communications Workers of America reached tentative agreement yesterday on a contract covering AT&T's largest division, but the nationwide strike by 155,000 union members entered its 25th day because of unresolved disputes in another division.

The AT&T strike had been tentatively settled June 17, pending completion of local contract negotiations at the company's six divisions.

But the unexpectedly complex task of consolidating 23 contracts from the former Bell telephone system into a single AT&T contract for those divisions has prolonged the strike more than either side expected, company and union officials said yesterday.

"A lot of the problem stems from the fact that we are still dealing with the aftermath of the AT&T divestiture" of 1984 when the government broke up the AT&T monopoly, company spokesman Herb Linnen said.

The final roadblock to ending the strike is the AT&T Information Systems (ATTIS) contract covering 35,000 technicians and marketing staff members who sell, install and service computers and business communications systems.

The ATTIS, which has many competitors, has been hard-hit by layoffs and transfers, prompting the CWA to seek more job security, better severance pay and other improvements.

Under the former Bell System, the 23 regional companies had varying rules on job titles, job security, transfers, vacations, overtime and other issues. In general, the CWA is seeking to preserve the best parts of those 23 contracts in the ATTIS pact, while AT&T is seeking to reduce costs, officials said.

Both sides said they hope to settle by week's end. At bargaining sessions in New Jersey, agreement was reached on the AT&T Communications contract provisions for 76,000 workers, including long-distance operators.