From the Orange website, always a better place to start
"switching off the Answer Phone
Sometimes you might not want callers to be able to leave a message on your Answer Phone. To switch off your Answer Phone call 150 for PAYM or 450 for PAYG.
If you want to switch off your Answer Phone for a short period of time, say for example, you’re going on holiday, then you can set up a temporary greeting and stop messages being left for you. To do this:
Step 1 Press and hold 1 to access your Answer Phone menu.
Step 2 Press 2 for Greetings
Step 3 Press 3 to set and temporary greeting and follow the prompts."