Repeating low airtime voice

I’ve always been happy with MTN, and I’ve had an account with them for ages. But recently while I’m speaking to someone, I get a continued voice that says “you only have enough airtime for a short call please load airtime” and it keeps saying that, so I can’t actually talk to the person on the other side. And when I check my airtime I still have R8 airtime. It’s a happened a few times where I just need to finish what I’m saying but can’t because of this voice prompt.