What to do if you received a call from a person claiming to be a lawyer

I received an unsolicited call on a Saturday night (6.06pm to be precise), on my mobile phone in my place of residence from a lawyer (think he's actually a partner) claiming to represent my ex-wife in a matter pertaining to a divorce agreement that was ratified in Zimbabwe at the end of August.

He threatened that the full force of his law firm will be thrown at me if I did not settle the monetary claim within 48 hours of our telephone discussion. 80% of the monetary amount in question was actually remitted that afternoon, prior to his call, which he didn't bother to inquire about.

There was no formal notification in writing that their services had indeed being retained by my ex-wife. I am not sure if he followed due process & his threats could be construed as coercion in a matter that was all but resolved.