I think the solution in this case is just going to be "drop AT&T" (which I respect as a suggestion for many reasons) but I still want to throw this out there in case anyone has had a similar experience.
Early last week, something mysterious occurred in telephone magic land and my in-laws are no longer able to call us from their land line phone. My wife and I both have iPhone 6's through AT&T. My in-laws have a land line through Wave Broadband, a relatively small provider on the west coast in the U.S. The subject number has not been blocked on either of our phones.
Here's what happens in our test cases:
In-laws call my phone or my wife's phone from their landline:
They either hear it ring forever and never reach our voice mail, or they don't even hear a ring. On our end, the call flashes up for a split-second and then says call ended, and it's counted as a missed call. As long as they attempt to connect, missed calls will tick off on our phones, about one every five seconds.
In-laws call anyone else, including my work phone, a cell phone through AT&T:
No problem.
We call our in-laws on their landline:
No problem.
In-laws call us from their non-Wave cell phone, generally only used in emergencies:
No problem.
Feel free to suggest any other test cases you think might shed some light, but I feel like we've really pinned it down to being an issue between our networks. Both carriers gave helpful enough customer service at first, acknowledging that it's an unusual issue and escalating help tickets to tier-2 support. Both tier-2 responses were "we can't find any problem on our end".
Any ideas?
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On my sleeve, let the runway start
Right -- they're in a pretty rural area and it wouldn't be reasonable to have them try to switch providers. They're elderly and not at all tech-savvy so the task of dealing with the carriers has fallen to me.
That's a good idea -- I don't know. I'll ask them!
My father-in-law recalled that this all started to go wrong when there was a confusing couple of seconds where my wife and someone else had called at the same time, and he had done some juggling back-and-forth between the calls using the "flash" button on the handset.
So, on the day that a Wave technician was coming out to the house to further diagnose this issue, my father-in-law picked up the handset and just pressed the "flash" button. Nothing obvious happened, but our issue was suddenly resolved. The arriving technician sort of shrugged and scratched his head, saying he'd never heard of this issue before.
What really bugs me about this as a solution to the problem though is that both my wife's phone and my phone were affected. My phone had nothing to do with the confusing call-waiting mix-up that started the whole mess. We are obviously on the same network, and my number is just one digit off of hers, but it's still a different number.
Anyway, I'm glad it's fixed but I'm still bothered by the fact that neither carrier has offered any ideas about what was actually happening. This can't be the only time this has ever happened to anyone, right?
Another step in your debug would've been to have them try to call you on their landline, but using a different handset/phone. It'd be only worthwhile to try when the issue is presenting.
Unless I'm misunderstanding the troubleshooting scenarios you walked through, it seems pretty clear here that neither of you nor your wife's phones were effected. The issue was with the handset your in-laws were using to dial out. I would not be surprised if they would have had issues dialing other numbers, too, and either didn't run into it or didn't notice it.
You may want to consider getting them a new phone set, too.
Well, keep in mind that neither provider is going to know the ins and outs of all handsets in the world. If you called the handset manufacturer they might have had enough cases to be aware of the issue, but a telecom would be unlikely to.
On my sleeve, let the runway start
Thanks for all the ideas.