provided by Star2Billing S.L.

I get the following error when I try to start the callback-daemon service:

Can't call method "val" on an undefined value at
/var/lib/asterisk/agi-bin/libs_a2billing/callback-daemon/a2billing-callback-daemon line 25

From this, it seems that it is not reading the a2billing.conf file, but all of the files, including the a2billing-callback-daemon.rc are updated with the correct location of the files.

Any ideas?

There's a syntax error in your a2billing.conf.
If you upgrade the callback-daemon to the one that ships with v1.3.1 or later it will warn you of this, rather than emitting that error message.