AirPods disconnect during calls only

I’m having a very annoying problem with my Airpods. They are constantly disconnecting and reconnecting whenever I am on a phone call. This happens all the time now. I use an iPhone 12 Pro Max on O2 sim. I do not have any issues listening to Netflix, YouTube, Spotify, Apple Music, etc. This only happens during phone calls. It happens with a single AirPod in an ear or both at the same time, indoors or outdoors, WiFi enabled or disabled.

I have tried restarting my phone, forgetting then reconnecting the AirPods and doing a reset, network reset, all suggestions on various forums. The AirPods are brand new and I replaced them ones thinking they are faulty. So far the issue persists and I’m having to use the wired headphones for phone calls. The AirPods work with iPhone 13 Pro Max which is used with Vodafone sim.

I used AirPods gen 2 for many years, never had this before. Could network provider have a bearing? Pleeeaase help.