When one of my contacts call me, they get an automatic response "invalid number" while anyone else who has tried to call me can get through without a problem. They are in the same city. They have an older flip phone. I have an iPhone 4s.
Do they have good reception in the city? Also are they on your blocked list? They may have been added to it by accident. It probably has something to do with their end though since everyone else can reach you.