Settings -> Phone -> Allow Phone to Access -> Incoming Calls -> Banner -> When Phone is Unlocked Display as -> Full Screen
Argh!! What did you do, read the manual?? Who knew it was an option.
One note though, "Allow Phone to Access" is not a tap on setting, but rather the heading for a section (that might be overlooked). Good catch! This solves kev's problem.
On a Mac since 1984. Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
Who knew it was an option.