MCI Communications Corp. has changed its advertisements to give consumers a better idea of how much it costs to use the company's discount long-distance telephone service.

The changes came after a division of the Council of Better Business Bureaus launched an investigation into MCI's ads, questioning some of the company's claims.

Among other things, the Council's National Advertising Division asked MCI to substantiate its claims that consumers could save 30 to 50 percent by using MCI's long-distance service instead of that offered by American Telephone & Telegraph Co.

The division, which monitors national advertisements to make sure claims are not deceptive, asked MCI why its ads and direct mail literature failed to alert consumers that on top of the charge for long distance calls, they had to pay a monthly $ 5 or $ 10 subscription fee.

The division also asked why the rate comparisons MCI made with AT&T long-distance charges were not made for the same comparable periods within the day and week.

After the inquiry was launched, MCI decided to revise its rate structure to be more comparable with AT&T's, and making easier comparisons between AT&T and MCI rates for the same evening time periods.

MCI also said its brochures and direct-mail literature now states that there is a monthly subscription fee and that consumers may incur a local phone charge for obtaining access to MCI's computer -- even when the number they are calling is busy or not answered.

MCI also promised to conduct an extensive computer survey among its subscribers to determine how much their customers save by not using the Bell System's long-distance service.

Gary Tobin, who is in charge of MCI's advertising, public relations and sales promotion, says the company plans to run customer endorsements in television advertisements.