Does divorce take effect and expiation become obligatory for a wife who spoke to her friend whom she swore not to speak to, given that the call came from an unknown number and while the husband was angry?

What is considered in breaking an oath is the intention and purpose of the swearer; for intention specifies the general and restricts the absolute. If, by your oath, you intended to absolutely prevent your wife from speaking to the woman, then you have broken your oath, and a divorce has occurred, unless it completes the three divorces. You may take your wife back before the expiry of her 'iddah (waiting period). However, if you did not intend to prevent her in a situation like the one that occurred, then you have not broken your oath. Some scholars, such as Ibn Taymiyyah, are of the view that the ruling on swearing an oath by divorce, which is not intended to be a conditional divorce but rather a threat, is the same as swearing an oath by Allah. In this case, the swearer must offer an expiation for an oath, and no divorce occurs. Since there is a difference of opinion on this matter, the correct course of action is to consult people of knowledge. One must be wary of swearing by divorce, as it is among the oaths of the wicked and may lead to undesirable consequences.