Is someone permitted to perform a "Melacha", such as answering the phone, if Shabbat is over but they have not yet heard Havdala?

The holiness of the Shabbat can be extended on to after Shabbat, in order to be able to do Melacha we should do Havdala, to that extent the words "Baruch Hamavdil Bein Kodesh Lechol" will be sufficient and you can now do Melacha even before Maariv or the Havdala on wine. (You can see more in Shulchan Aruch Orach Chaim 299 and Mishna Brura 40 ibid)