Xile:
So I tried my SIM in a different phone and had no issues.
It would appear that Oppo phones have a history of cutting out during calls as they have a web page with possible fixes on it: https://support.oppo.com/nz/answer/?aid=neu6126
None of which helped with my phone. Lodged a fault with Oppo, waiting to hear back from them.
Was the other phone using VoLTE? If not you aren't comparing apples with apples.