Does the phrase "I will make you swear an oath not to hang up the phone on me" or "He made you swear an oath if you hang up the mobile on me again" count as a conditional divorce, even if the husband did not utter the word "divorce" but only the word "oath"? And is the wife sinful if she does not ask him about his intention?

What the husband mentioned about a promise to swear an oath, whether by Allah or by divorce, does not result in divorce or expiation merely by virtue of it. Nor is the wife obliged to ask him about his intention. It is advisable to turn away from these whisperings and get rid of them by seeking Allah's help and making abundant supplications.