There should be a factual answer, but mods, if it needs to be moved, so be it.
About 6 months ago, my daughter was complaining that half the time she calls me on my iPhone 8, it doesn’t ring and goes directly to voicemail. When I finally get in contact with her, my phone doesn’t show a missed call or a voicemail. I assumed it was a problem with my old phone or my service provider (Spectrum Mobile). It could happen no matter where I was at the time, and I typically have good connectivity in my area, at least 3 bars.
Then, about 3 months ago, it started happening when I called her. She uses AT&T, but it only happened when she was at home. I would call her, and it wouldn’t ring; it would go directly into voicemail, and she would have no record of an incoming call or voicemail on her phone. She has an iPhone 17.
Last month, I broke down and upgraded to an iPhone 17, and for a while, the problem seemed to go away. Unfortunately, starting sometime last week, the problem returned whenever she tried to call me, and no, I don’t ever have my phone set to DND. I haven’t noticed the problem when I call her yet.
The solution to these types of issues usually involves some combination of rebooting your phone, making you have the latest version of iOS, and resetting network settings.
But see here for official guidance from Apple:
With respect to Item #3 in the attached for iPhone settings, I discovered a few years ago that I had accidentally blocked my sister’s number. What’s worse is that you receive no notification whatsoever when they call or text you after that, nor can they leave you a voicemail. And they receive no notification that you have blocked them. It’s a great tool to block spammers, but not so great if you accidentally block someone you want to hear from.
It seems to be almost exclusively between my daughter and me, which makes this problem even weirder. Of course I call her, or hear from her, almost every day
Do either of you have Wi-Fi Calling turned on? If so, I’d try turning it off.
Settings > Celluar > Wi-Fi Calling > Off
Also, assuming you’re both using 5G, what happens if you both switch to LTE? Does the problem persist?
Settings > Celluar > Cellular Data Options > Voice & Data > LTE
In the Phone app, go to Recents, then tap the hamburger button at the top right. Tap “Unknown Callers” if it is available. Are her calls listed there?
Since this kind of sounds like an unknown caller kind of problem, I would suggest you both delete each others contact in your respective phones, then make fresh new contacts for each other.
edit to add
What happens when you trying messaging each other?
I have similar, periodic but recurring, problems with my Android phone and to/from other phones of different types and carriers. I live in a semi-rural area where reception is mediocre on the best of days.
As best as I can guess, it might be due to the flakiness of wi-fi calling and cell technology in general — but not necessarily on OR off. With wi-fi calling on, it seems like the handoff is unpredictable and can sometimes boost and sometimes degrade connection quality, especially if it has to constantly switch between cell and wi-fi near the boundaries of each, like when you’re just leaving home and right at the limits of your router’s range. But even with wi-fi calling off, you’re still subject to the whims of cell towers and RF interference, with all of its own unpredictability.
The only way I’ve really found to bypass this is to:
Use a phone connected to my stable home Wi-Fi connection (i.e., not shitty coffee shop wifi or cellular)
But placing an internet-only call, e.g. over Discourse or WhatsApp or Google Meet or such, NOT a regular phone call or FaceTime call (which will automatically try to negotiate between different protocols and connections)
Doing it this way deliberately takes away the “intelligence” that the programmers try to force on you and thereby prevents the auto-negotiation between different signal paths that your phone would otherwise try to do. It forces your voice traffic over a “known-good” path (the basic internet) not as subject to the carrier’s whimsy — especially the budget MVNO carriers like Spectrum, who don’t own their own networks and face unpredictable levels of network de-prioritization from their parent cell companies.
Oh, how I miss (pre-VOIP) copper landlines… still nothing today comes close in quality or reliability.
I have a Call Blocker app on my iPhone 17 from my cellular provider (rhymes with “horizon”). It occasionally blocks calls from numbers in my Phonebook! When I clicked a button for an explanation it said the number has characteristics of Spam Callers. It gave me the option to confirm it was legitimate. Sheesh. Being in my phonebook isn’t sufficient?