Selective call bearing

There are certain numbers that cannot get through to me via voice call. they always have to send me sms/call backs/whatsapps or even call me from other numbers. These numbers are numbers that were able to call me before and now they can't. Furthermore is that there are many other numbers that never called me before and cannot get through to me, this has lead one to wonder how many business opportunities one has lost due to this.

I have called them to resolve this issue and they can't. i have changed devices, done sim swaps, called them but they can not solve the problem. i do not know what else to do.