He had a duty to the American people to maintain some log of his calls, versus circumventing the process like a drug dealer using burner phones.
Why not log his calls, especially on that day of all days?
From Knute's link below:
"Multiple sources have described the record-keeping during the Trump administration as generally chaotic. Those sources, as well as witnesses who have testified as part of previous congressional investigations related to the former President’s conduct described Trump as deeply suspicious of the White House switchboard and detailed various ways he sought to avoid having records of certain phone calls from being kept.
A Senate Intelligence report from 2020 includes witness testimony from former aides saying that Trump regularly used the cellphone of his body man, Keith Schiller, to place calls to Republican operative Roger Stone because he did not want his to advisers to know they were talking.
“Trump hated people knowing who he spoke to, including from the residence at night when they went through the switchboard,” one former Trump official told CNN.
When John Kelly was Trump’s chief of staff, he monitored the switchboard to see who Trump was talking to. Trump would often tell people to hang up and call him back on his cellphone if he didn’t want it showing up on the switchboard call log.
When Mark Meadows took over as chief of staff, sources said he restricted the number of White House officials who had access to the call logs, limiting the group to only a small number of top aides."