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ISBN book numbers are non-transferable. A company that tells you that they are "assigning" you an ISBN book number and that you "own" that number is lying to you (this is a favorite trick amongst the vanity/subsidy publishers). They assign a book a "unique" ISBN book number but that number is not owned by the author, it is owned by the vanity/subsidy press.
If you use one of these ISBN numbers and then go to change publishers/printers/publishing services, or enter into self book publishing, you will have to start all over and get your own ISBN book number from the only place licensed to sell ISBN's - RR Bowker.