Not sure if this a 2Degrees fault, or a phone fault but it's annoying.

This is the situation:

Every two weeks I make a phone call to a landline number in Whanganui using my Oppo Find X3 Pro on 2Degrees. I'm calling from Napier. The call can last up to an hour. Every 9min 28sec the call cuts out, or disconnects. After calling again the call will last for another 9min 28sec before disconnecting again and so on.

I've only had the Oppo for maybe a couple of months. Prior to that I had a OnePlus 7 and had no issues, so I'm suspecting it's the Oppo. But what would it be? I've had a look through the settings but can't see anything which might cause this.

The Oppo is running ColorOS V11.2 with Android 11.

Any ideas?

Thanks in advance