The first 3 lines of code after '[incoming]' should have 's' as the extension, instead of '123'.
So they should read:

exten => s,1,Answer()
exten => s,n,Background(enter-ext-of-person)
exten => s,n,WaitExten()

According to paragraphs earlier in the chapter, 's' is the extension used when calls aren't
calling an actual extension (such as those coming from FXO channels). So 's' needs to be there to
handle correctly calls coming from the outside/trunk line(s) - at least this is what the author
professes his/her dialplan is supposed to be doing.

It seems to me that the mistake started somewhere on page 129, after the author providing
examples using extension '123' for several pages before, all of a sudden extension '123' appears
where extension 's' use to be in the example dialplan. If the use of extension '123' is
intentional in the final plan, it is certainly downright confusing, and doesn't have any prior
explanation for its use in the dialplan.

the:
CREATE FUNCTION loin (cstring) RETURNS lo AS 'oidin' LANGUAGE internal IMMUTABLE STRICT;

returns an error:

# psql -h 127.0.0.1 -U asterisk
Password for user asterisk:
Welcome to psql 8.1.11, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit

asterisk=> CREATE FUNCTION loin (cstring) RETURNS lo AS 'oidin' LANGUAGE internal IMMUTABLE STRICT;
ERROR: permission denied for language internal

The database and permissions where all set according to the previous sections.