The year of 1978 heralded a renewal of Zodiac letters on April 24th 1978 and May 2nd 1978, so the mention of a phone call in July 1978 wouldn't have been that unusual. After all, somebody claiming to be the Zodiac Killer mimicked the yet to be received April 24th 1978 "I am back with you" letter during a phone call on March 13th 1978 - received by a man who lived in the Mission District of San Francisco. The phone caller stated "This is the Zodiac. Tell the press that I am back in San Francisco". An unauthenticated third communication was mailed from Los Angeles to the San Francisco Chronicle on July 19th 1978, beginning with "I am the Zodiac and I am in control of all things" - which employed the use of the word "control" twice, in extremely similar fashion to the 1969 Melvin Belli letter. So, the timing of the phone call in July 1978 may be relevant to this letter, whether the Zodiac Killer or not.
The newspaper article is so badly written that it has seemingly fused two events from 1969 and 1978, although it would be difficult to comprehend how an article written in 1981 could confuse July 1978 with the events of October 22nd 1969, in which a Zodiac impersonator rang the KGO-TV station and spoke with Jim Dunbar and Melvin Belli. Also, the article mentioned that the "Zodiac Killer" asked for a meeting at a television station rather than just making a phone call to one.
The only other way of interpreting this newspaper article, is that Melvin Belli spoke in 1978, recalling the events of 1969, when somebody claiming to be Zodiac rang the Jim Dunbar Show and later asked to meet the attorney, but police swarmed the area. Although, the Zodiac Killer wasn't claiming 13 victims in October 1969.
It would be a far more interesting story if we knew that the Zodiac Killer had rang Melvin Belli in July 1978 and renewed his contact with the famous attorney, but unfortunately the layout and wording chosen by Linda Field has made it impossible to decipher with any confidence.