True, DJI even recommends your mobile phone be at least 1M away from the drone during calibration.
Things that people often miss:
-metal rebar in concrete surfaces
-metal framed buildings
-cars (see me other thread, wow!)
-shallow buried water pipes
-having an electromagnet in your pocket (just kidding)
I live in a wood frame home, but just below my main floor is a steel beam which supports the house. If I calibrate near it, I get errors.
It may be inconvenient, but go to the middle of a sports field somewhere. Getting a good compass calibration is essential, and will lead to a much more pleasant flying experience.