Can't call Nokia 110

My son has a 'dumbphone' Nokia 110 on Lebara; and for over a month now, when I call it, it rings once, then cuts off and says 'User Busy'. I also can't send him basic text messages. Before this, I could and it worked fine. Everyone else can call and message him fine. I've checked his phone and my Smarty number isn't blocked, its not on call forward or anything. I've looked on loads of forums and they suggest the problem is likely with my phone/ service. Can any one help? I don't want to have to quit SMARTY which I otherwise find to an excellent service, but I will have to if I can't contact my son! Thank you!