OT: What is a nnnn.7z voice message?

[joe at actionline.com]

Recently, I've begun to receive email messages like the one
shown below with a "nnn.7z" attachment. Anybody have any
idea what these are or how to read them?

Message from 02028877960
From: Voice Message Server <server at 9138317697.tlnf.com>
Date: Wed, October 18, 2017 at 7:30 pm
To: (one of my email addresses)

19/10/2017, 08:15:35 PM
65,{rndnum(1,1)}}-second message deposited by 02028877960
Voice Message(02028877960.7z