/etc/logcheck/ignore.d.server/isdnlog is in logcheck-database 1.3.17.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | ^\w{3} [ :0-9]{11} [._[:alnum:]-]+ isdnlog: | line.* : free$
^\w{3} [ :0-9]{11} [._[:alnum:]-]+ isdnlog: | line.* : outgoing$
^\w{3} [ :0-9]{11} [._[:alnum:]-]+ isdnlog: .* outgoing .* -> .*$
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