I found no instruction on how to use the “dump” file.
However the “db” files in the same backup are usable.
I followed this:
Import selected SMS from a backup [closed] - together.jolla.com (2015)
My commands (on the PC where the backup has been copied to):
sqlite3 commhistory.db
.mode insert Events
.output foo.sql
select * from Events where type = 2
It produces a list of list of SMS in clear SQL format. If you know SQL language you could filter by phone number already in the command, otherwise you can filter by later searching the final text file.
Side effect is MMS are lost.
Other parts of the documentation said there are SFOS commands to do this:
https://together.jolla.com/question/171073/restore-only-call-log-and-message-history-from-backup/ (2017)
How to restore backup from another device? (2020)
commhistory-tool export -calls calls.dat
It did not work for me (produced an empty file) but you could try.