Why do Dutch websites require Dutch format phone number?

Many Dutch websites seem to require a 10 digit Dutch phone number, whereas in reality, any valid phone number would be fine for the actual process.

Some examples of websites that require specifically a Dutch phone number where any EU phone number should be fine, but is not accepted due to the coding of the website:

http://www.ikea.nl
http://www.afvalstoffendienst.nl/
https://www.ziggo.nl/
https://www.tele2.nl/

At the moment, my experience has been that if a phone number is required on a Dutch website, it with 99% probability requires a Dutch number, whereas for the actual process either my Finnish or Swedish number would be entirely fine. It's just the damn website that doesn't accept the number!

My question to the Dutch webdevs is: Why do you require a Dutch phone number? Why not any EU number, or, to think of it, any valid phone number?