A Call from Frontier Advisory

Today I received a call from Frontier Advisory in Johannesburg, South Africa. I was in Taekwon-Do training at the time and missed the call. I'm very curious about the call because it was made to my landline number in South Africa that is automatically redirected to my phone here in Korea. It is an unlisted number and a very specific group of people have the number and none of them works for Frontier Advisory. Also, it is not the type of number one would accidentally call as it is not a typical land line number. I returned the call, but the phone operator could not help me determine who the call was from.

Here's what I'm wondering: How did they get my number? And why would they call me?