NEW YORK (AP) _ Unionized telephone workers began returning to work today after negotiators agreed tentatively on a contract with NYNEX Corp., ending an eight-day strike by 38,000 workers against the New York and New England telephone companies.

The Communication Workers of America and NYNEX issued a joint statement on the tentative settlement following ''intensive bargaining'' during the weekend and Monday.

NYNEX was the last of the seven regional Bell phone companies to reach a tentative settlement with its major unions. The only CWA members still on strike against a Baby Bell company are about 380 employees of Ameritech Publishing Inc.'s Ohio unit.

Although the NYNEX employees returned to work today, a date for a ratification vote had not been set, said CWA spokeswoman Clara Allen.

Jim Crosson, a New York Telephone Co. spokesman, said a verbal agreement between the company and the CWA is pending review by local officials of the union.

Tony Pappas, another New York Telephone spokesman, said this morning that the workers were back on the job.

''Everyone who should be here is here,'' Pappas said. ''Of course, there are always a few stragglers, but it's business as usual.''

Neither Crosson nor CWA spokeswoman Clara Allen would discuss the terms of the tentative three-year settlement. The joint statement said it reflected ''a tough bargaining process.''

Union sources told The New York Times that the sticking point in the talks had been the issue of health benefits.

About 37,000 CWA members in New York and 1,000 in New England struck after their contract expired at midnight Aug. 9. Most New England Telephone workers belong to another union, the International Brotherhood of Electrical Workers, which settled that weekend.