Multiple iPhones on same Apple ID not ringing for incoming calls

I am asking this for my friend in the UK. She has four iPhones on her Apple ID; an iPhone 16 pro max, an iPhone 14, an iPhone 12 Pro, and an iPhone 11 Pro Max. All have auto updates on for both iOS and apps, and all are on the latest iOS they can run. 

The iPhone 12 and 14 are used for business, the 16 is her personal phone and the 11 has a foreign (Chinese) sim for when she travels. The main business phone is the 14. All the UK phones are on contracts from Three UK. 

She sometimes has customers calling on the 14, but the phone doesn’t ring at all. However the call does show in the missed calls list. Some calls come through randomly. This also happens randomly with the other phones, but their volume of calls is much less, so harder to tell. Some of the calling customers are in the contacts list (return customers) and some are new and unknown. It seems random when some are silenced.

Even weirder, the missed calls sometimes show in the list on the other phones, randomly. And weirder still, when her friend had the two work phones here in UK, but she had travelled to China with the 16 and the 11 it was still happening. Some of her Chinese calls showed on the missed calls of the 14 and 12 in the uk (the phones didn’t ring however) but the calls were directed at the 11 with the Chinese sim while in China. And on the 11 and 16 in China she was getting missed calls from the 14, but in all cases like this NONE of the phones ring. 

I just want to make clear, none of the phones have any call sharing turned on in any way. They should all be separate. She keeps them all on the same apple ID because she pays for storage for the one account, and she also wants to share photos, calendar, contacts, and notes between the four phones. 

We have checked the settings on all four phones; WiFi calling is off, calls on other devices is off, call forwarding is off, silence unknown callers is off, respond with text is off, none of the callers are blocked, we even turned off the network answering service. This has been happening for a while, even when the phones were on O2 network previously.

It’s nothing she is specifically doing herself; when her friend had the work phones the same issue was happening, and I have seen it happen firsthand. The screen doesn’t even light up or display a caller, the first you know that there has been a call is when it shows in the missed calls list. 

Any thoughts anyone please? 

[Re-Titled by Moderator]

Original Title: Calls not sounding on four phones on same Apple ID, showing in missed calls list of multiple phones.