Modelo 347 is a form all businesses or autonomos who have sent or received invoices to/from other businesses over a certain amount in the past year, to verify both parties have declared the same amounts of IVA. You used to contact one another to make sure they are 100% correct, but now large businesses have the obligation to register their IVA electronically, directly with the Tax Offices, so that´s why Endesa e.g. no longer submits it to clients in paper.
The window to submit the form 347 in 2020, so over tax year 2019, is from the 1st of February to the 2nd of March 2020.
More info on the website of the Agencia Tributaria/Tax Office.