MCI Communications Corp. this month starts a public relations effort for its 1-800-COLLECT discount long-distance collect calling service, target ing bewildered parents of new college students. MCI is distributing thousands of copies of an eight-page brochure offering parents advice about what to tell their departing offspring. Topics include tips on choosing roommates, budgeting, study habits and calling homevia 1-800-COLLECT, of course. Parents can call a toll-free number to get the brochure, which is being publicized through various media outlets this fall. The promotion was handled in-house.