I am not sure of your claim that calls made on n28 band are VoLTE.
I believe they are Vo5G because n28 and n78 are used purely for 5G and since it’s Standalone deployment, calls made on such network are Vo5G.
Your hardware engineer will have no knowledge of software used for making calls. Rooted Pixel with Network Signal Guru app shows that call type is Vo5G.
All Vo5G calls will be made on n28 band due to it’s penetration power.