When entering a VAT number to your Company Details, Hotjar is legally obligated to Validate them before storing them in our database. To validate them, we use the VIES API which is provided by the EU:
Hotjar currently only accepts intra-Community VAT numbers which are successfully validated by VIES. Legally we are bound to charge VAT to European customers who do not provide one such VAT number.
VAT Validation issues
If the VAT number you enter never validates, please try validating it in the URL below. Chances are that either the API is not available or the VAT number you have is not an intra-community one: http://ec.europa.eu/taxation_customs/vies/vatRequest.html
Since we rely on VIES to validate VAT numbers and are legally obligated to do so, at times VAT validation fails if the API is not available.
For frequently asked questions related to VAT number validations and the formats each member state prescribe, you can find the FAQ here: http://ec.europa.eu/taxation_customs/vies/faq.html#item_11