This phone call was captured as a recording on a voice answering machine, but I suspect it was routinely dismissed as a hoax as many phone calls had been previously and likely lost to the hands of time, despite no evidence for such a conclusion. Much evidence has fell by the wayside through incompetence and lethargy in the Zodiac case, and this phone call is just another example of a missed opportunity.
On May 5th 1978, just 11 days after the "I am back with you" letter, somebody claiming to be the Zodiac Killer phoned the San Mateo Police Department and falsely reported that a bomb had been placed in the Transamerica Pyramid at 600 Montgomery Street. This too had a loose connection to the 1978 letter, which mimicked the Melvin Belli letter mailed on December 20th 1969. Both the 1978 and Melvin Belli letter began the introduction with "This is the Zodiac speaking I", which were not only grammatically incorrect in both instances by failing to punctuate between "speaking" and "I", but were the only two communications to date that didn't keep the "This is the Zodiac speaking" introduction exclusively on one line. Something a copycat would have been expected to do, had he copied at least four Zodiac letters published in the newspapers. An image of the Melvin Belli letter has yet to be discovered in any newspapers up to April 24th 1978, making this grammatical anomaly difficult to explain from the standpoint of a copycat. The 1978 and Melvin Belli letters also inversely mimicked each other by the use of "I can not remain in control for much longer" in 1969 (one example) and "I am now in control of all things" in 1978. Bearing in mind these comparisons between the two letters - and the Melvin Belli letter was mailed to 1228 Montgomery Street in San Francisco - it is unusual that the telephone threat on May 5th 1978 was directed against the Transamerica Pyramid at 600 Montgomery Street, just 600 meters south of the Belli residence. The law offices of Melvin Belli at 722 Montgomery Street, purchased in 1959, were even closer to the Transamerica Pyramid at a mere 100 meters.
When the Zodiac Killer last claimed he was to plant a bomb on June 26th 1970, he utilized the peak of Mount Diablo on the eastern side of San Francisco Bay in Contra Costa County, which dominated the skyline at 3,849 feet. When the Transamerica Pyramid was completed in 1972 it was the tallest building in San Francisco, with an elevation of 853 feet. Would it really be a surprise if the Zodiac Killer had chosen the two most prominent landmarks on the San Francisco Bay Area skyline to threaten its residents with bombs? The Transamerica Pyramid would also feature in the 2007 Zodiac film directed by David Fincher, shown in time lapse photography to depict the transition of time between Zodiac events.