MCI Communications Corp. will start charging residential customers a fee to help pay for low-cost Internet hookups for schools and libraries, affordable phone service for the poor and other government-mandated subsidies. In a filing with the Federal Communications Commission, Washington-based MCI said it will levy a 5.9% charge on residential customers' long-distance interstate and international calls starting July 1. The nation's second-largest long-distance firm also will increase the fee it has been charging large business customers for the government programs, to 4.9% from 4.4%, effective Aug. 1. Small-business customers--those with bills totaling roughly less than $500 a month--will be assessed the same percentage fee as residential customers beginning in July. AT&T Corp. announced a similar fee last week, touching off a protest from FCC Chairman Bill Kennard that it was unwarranted. The FCC estimates that providing discounted Internet hookups costs less than $1 per line per month.