Warranty

Don't let them figure it out. You can get a direct diagnostic service (most likely remotely) from Apple. I'd recommend an Apple Store for an in person visit, but they're not available in Ireland. Or at least not the Republic of Ireland.

From the Get Support link (Start Now) at the bottom of this page, you can select Connectivity, then "Calling, voicemail or other phone functions not working as expected". The options I see (not sure if they might not work for you) are Contact Carrier, Bring in for Repair, Chat, or Schedule a Call. It sounds like you exhausted your options with the carrier, so maybe a chat or a call. They should be able to run a remote diagnostic service as long as you have good internet access through Wi-Fi. I believe it goes to a special website (I've only done this in an Apple Store where it had to be via their store Wi-Fi, so it may be different remotely).

iPhone - Official Apple Support

You are not writing Apple here. This is a user to user forum, which Apple has little involvement. Use the Get Support Button at the top of this screen to open a dialog with Apple directly.