This first came to light a few months back when I phoned my local Chinese to order a meal. I've used the same number for 20 years.
As the first number rang out I tried their second second number which did the same, tried a few nights later and still the same, thinking maybe they were shut for some unusual reason. Drove past later on in the week and saw it was open and phoned that evening but still rang out. Phoned using my mobile and got thru' straight away.
I've since asked if the have the facility to block incoming calls, to which they don't but have said they have been getting calls from people who say they have called them when they haven't.
Obviously a network fault which the business says they've reported to no avail.
If a team member could PM me I will forward the business details.
Sorry to learn about the problems you're experiencing, I'd like to take a closer look at this from our end for you.
It's worth mentioning that it's more likely an issue with the one line, rather than a national issue only affecting one number, so I'd recommend the Chinese get in touch with whoever their provider is too.
That being said, I've sent you a private message, requesting some information so I can be sure nothing is wrong from your end.