Telephone exchanges throughout Cambridge--including Harvard's Centrex exchange--broke down yesterday, leaving the city without phone service for nearly half an hour.

The malfunction prevented Cambridge residents from making phone calls between 3:15 p.m., and 3:40 p.m., but did not prevent them from receiving calls.

Susan McEnany, business office manager for the Cambridge branch of New England Telephone Co., yesterday described the problem as "Intermittent dial-tones," Company officials have not yet determined the eause, she added.

McEnany said the malfunction was "a major service problem," but added that the inconvenience experienced by consumers was "not nearly as bad as it might have been."