You have to use querySelectorAll and then loop over the returned collection:
const spans = document.querySelectorAll('span');
spans.forEach(span => span.addEventListener('click', (event) => {
if (event.target.innerHTML === "Zavolajte nám ihneď") {
event.target.innerHTML = "+421 911 111 111";
} else {
event.target.innerHTML = "Zavolajte nám ihneď";
}
}));<span>Zavolajte nám ihneď</span><br><br>
<span>Zavolajte nám ihneď</span>querySelector only returns the first element within the document that matches the specified selector, or group of selectors. If no matches are found, null is returned.