The same thing happens to my wife. We are in Portugal at the moment and for the last couple of calls from our son in Switzerland have gone straight to her voicemail so he contacted her on WhatsApp. A call from her sister in the UK said the number is unavailable. Our daughter calls her everyday from the UK and gets through every time with no problem. They all have their names & numbers saved in her phone. The handset is fully up-to-date has plenty of memory and they are not blocked as here sister got through the following day! This has also happened when we are in the UK with other people both listed in her address book and not listed i.e. number unknown.
