it is not necessary to announce your chamber of commerce number. but for everyone who wants to know just search for "FBTC" in
http://www.kvk.nlIt may or may not be "necessary", but it sure is good business practice. (EDIT: after reading the dutch thread, it seems it IS necessary according to Dutch law to give your KVK number in all company communications.)
I did a search on kvk.nl Handelsregister for "FBTC", and only a company in Woubrugge, NL came up. But your site has contact as Contactweg 131, 1014BJ, Amsterdam, NL.
Where are you located? Do you have multiple office locations? What is your KVK registration number? Do you have multiple KVK registrations?