Is it possible to get an accurate population count without an official census?

Census is more official, and doesn’t require proprietary customer count data from private companies.

Not everyone has a cell phone (elderly, people in nursing homes), and some people have multiple cell phones, so that’s not an accurate measure.

No

Accurate approximation is an oxymoron.

Also, people do not have the same access to resources (cell phones, utilities, etc.).

approximation – close to the real number, accurate – very close

accurate: close in all details, exact

approximation: a value or quantity that is nearly but not exactly correct

Per https://www.merriam-webster.com/dictionary/accurate

Free from error, especially as the result of care: an accurate diagnosis.

Conforming exactly to truth or to a standard: exact: providing accurate color.

IMO, approximations can be accurate: CA has almost 40 million people.
However, it would be inaccurate to say that CA has exactly 40 million peeps.

Mic drop…on foot; I think it would be accurate to say that my foot hurts. :p

Hmmm…I can see both sides of this debate. I think my brain needs more sleep. :P