Afrihost: how to use "free" MTN airtime?

[ros_b]

I am a pre-paid Afrihost customer. I have some Afrihost airtime, and then the other day, when I "woke up" my Sim card after a week of being overseas, I got a message from MTN that I had been granted R50 airtime valid for 3 days. So I've been merrily using my Afrihost sim to make calls, assuming I was using up the R50 that expires in 3 days. I just checked and I see the full R50 is still there - instead I've been using my Afrihost airtime (that doesn't expire!). So how can I make my phone use the R50 "Wake up tour SIM" airtime?

 

[iCuSD]

That 50 Rand might be used for mtn - mtn calls or it will use up the afrihost credit then dip into the bonus