One contact in iOS, two different phone numbers

My girlfriend and I both have an iPhone for personal use.

To be precise, I have three Apple devices:

1. iPad Air (4th gen) —> private use, shares the same iCloud account with iPhone 16 Pro Max, no SIM card
2. iPhone 16 Pro Max —> private use, shares the same iCloud account with iPad Air (4th gen), personal SIM card
3. iPhone 6S —> business use (has its own iCloud account), business SIM card

My girlfriend simply has one Apple device: an iPhone 15 Pro Max registered with her own iCloud account.

All devices’ iOS are up-to-date.

Since October last year we moved from WhatsApp to iMessage for our communication.

Now, lately we are experiencing some issues when using iMessage.

My girlfriend has registered me as one contact person on her phone. Both, my personal SIM and business SIM are registered under the same contact person. Super easy.

The problem is that her iPhone seems to switch message delivery from one phone to another like super randomly.

On the web I found an easy way to create a shortcut, allowing her to open up our iMessage conversation by a single tap. On the Shortcuts app on her phone I created a shortcut. It is configured to run the tel: protocol (appended my personal phone number to it), added it to the Home Screen and pinned it to the Dock on the bottom of her screen. This way, she opens up our iMessage conversation the same way, always.

Yesterday, I didn’t seem to deliver her iMessage. Until tomorrow, I started working so I turned on my iPhone 6S, and there it showed op the missing iMessage.

I’m really confused. I’m an IT guy myself as well, but I can’t seem to figure out what’s going on.

A couple of weeks I tried contacting Apple Support. They did a diagnostics on my phone and all seems well. I didn’t do anything strange.

I tried disabling iMessage on my work phone (iPhone 6S), as I don’t use it for work anyway. Still, it results in the fact my girlfriend’s phone randomly switching ‘recipient’.

I’m clueless what to do in order to fix it.

Any help would be very appreciated!