How can a hacker win by hacking a VoIP phone?

Hi, I worked in a place where they handled VoIP services.

I heard a friend who works there today that a hacker gained the power of his entire network and managed to call several countries using more than 6000 VoIP lines.

I mean, I think the company will have to pay for that and that stinks, but will the hacker benefit from that feat? or is it just a way to harm the company?

??

Thanks for your time 🙂