What is the ruling on someone who swore an oath by divorce not to answer a call from a particular person, but then answered them from another number, unaware of who it was? Does his divorce take effect?

As long as you answered the phone unaware that the caller was the person you swore not to answer, then you have not broken your oath. Sheikh Ibn Uthaymeen said: "If someone swears an oath not to do something, then does it out of ignorance, such as swearing not to speak to a person, then speaks to someone not knowing that he is the person he swore not to speak to, then he has not broken his oath." It is important to avoid swearing by divorce; the permissible oath is to swear by Allah the Almighty.