you can have an interceptor put on the phone gloop, BT will help you with this, they will intercept each call till they get to this one then they have ,or did have ,a internal body who would be able to trace it and they could be prosecuted too, another option after that would be for you Mum to go on a register that would block all this type of of call like the overseas call centre ones, what happens there is, from a computer a random number of calls will be sent out, eg, 10,000, but maybe there are only a couple of people doing this, hence the silence when you pick the phone up, again BT will advise you, hope you get it sorted