Hello,
One thing you should know is that dialing a US toll free number from abroad is not free. If you want the call to be free for people in the Netherlands you must use a Netherlands toll free number that redirects to a US phone. Another option is to use a Netherlands local number also redirecting to the US (in both cases you, the receiver of these calls would pay international charges). Two such services that come to mind are Local Phone and United World Telecom for Dutch local numbers and toll free numbers respectively. Below are links to these services:
https://www.localphone.com/services/incoming_numbers
https://www.unitedworldtelecom.com/services/toll-free-numbers/
To answer the other part of your question: to call a US toll free number from the Netherlands the following dialing sequence is required: 00 1800 xxx xxxx. Note that some US toll free numbers can not be accessed from abroad. This usually happens when the toll free number’s owner choses not to be contacted from other countries which makes sense since calling the toll free number is not free for the caller.
If you have any further questions please reply to this message.