Telefonica – sell the stock before exDividend date in order to avoid Spanish tax on dividends
Spanish dividend tax (Quellensteuer) is 30% and although within the European Union one can claim a half of it back, it practice it very hard to get it back. Since the exact ex-Div-Date is yet unknown (but expected to be in May, 2018), keep checking https://www.telefonica.com/en/web/shareholders-investors/share/dividend-history