Cannot receive calls

Hi, please assist my Father he has been struggling with this issue for weeks now, he cannot receive calls. He has to phone people and that is frustrating because sometimes he doesnt have airtime or we want to reach out to him but we cant, we can only send him messages and if we lucky and he has airtime he will call. Please i urge you guys to help him out with this issue urgently. Thank you in advanced. His number is ***.