fxotune -s is generally run by /etc/init.d/dahdi on startup if fxotune.conf exists, FYI dahdi is not part of asterisk nor FreePBX so please adjust your thinking.
No you don’t need another asterisk box you need as local as possible (relative to your pstn provider) milliwatt test number, google for that. Call it and when you get the connection transfer to 88888 where you have something like
[custom-testtone]
exten => 88888,1,Answer()
exten => 88888,n,Milliwatt()
in your extensions_custom.conf
use dahdi_monitor -vv on that channel to read levels, without being too technical it should be at about 70% and exactly equal for both, adjust rx gain to match the tx, I believe you will need to RESTART not RELOAD dahdi and asterisk each change, but maybe that is fixed.
I’m sure some one can supply a callfile or an asterisk CLI “originate” line to do the same but I won’t get into that here, because it’s well documented but nobody ever reads the documentation and I am not prepared to read it to them 