If the customer does not appear in VIES, the safest option is to invoice with Spanish VAT until they sort out their registration.
Joining the ROI is not automatic when you register as an autónomo or set up an SL: you must request it explicitly in Modelo 036 and the tax agency may review the request before granting it. Until you are listed, your own EU VAT number, which is your NIF with the country prefix, does not show as valid in VIES.
Typical mistake: checking a customer's number once and never again. A number can be deregistered, so it is worth checking before the first invoice and from time to time, and keeping proof of each check. Another slip is assuming it is enough for the customer to be a business: without a valid number, the sale is not treated as intra-Community.
Being in the ROI also means filing the recapitulative statement of intra-Community operations, on top of including these invoices in your VAT return and registers. In kontora, when you invoice an EU customer without VAT, the invoice carries the required note and the transaction flows into the returns that report it.
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