Call a batch file with variable?

I'm doing

call %ANDROID_SIGNER% sign -v ...etc

where ANDROID_SIGNER is

set ANDROID_TOOLS_PATH=%ANDROID_SDK_PATH%\build-tools
set ANDROID_TOOLS_VER=29.0.2
set ANDROID_SIGNER=%ANDROID_TOOLS_PATH%\%ANDROID_TOOLS_VER%\apksigner.bat

etc

and I get Unknown command "C:"

Am I doing something wrong?

Thanks