AT&T Corp. and Mexico's Telefonos de Mexico, the country's largest telephone company, have signed an accord to lower a cross-border interconnection rate to make it cheaper to call between Mexico and the U.S. The new rate, which covers the cost of completing a long-distance call between the two nations, will drop to 19 cents a minute from 37 cents. It becomes effective July 1. Telephone traffic between Mexico and the U.S. is the second-busiest in the world between two countries and is expected to continue growing as the two economies become even more integrated because of the North American Free Trade Agreement. The new cross-border rate, which is also expected to discourage other telephone carriers from trying to bypass the two companies' phone networks, will last until 2000.