Does a divorce take effect with the words "I divorce my wife if the phone is off" if it turns out that the phone was not off, but rather out of service?

If a husband swears an oath of divorce, believing what he is swearing about to be true, but it later turns out to be otherwise, then the divorce does not occur. It is considered a 'laghw' (unintentional) oath, because he did not intend falsehood; rather, he spoke based on his assumption. The husband should avoid swearing oaths of divorce.