Called Vodafone's 0800 support and it says they couldn't detect the mobile number I'm calling from, so I should enter it. I enter it, then go through to customer support where the first question they ask is "What's your mobile number?".
you enter a vodafone number so vodafone knows its talking to a valid customer as to not clog up their phone lines. you may be calling regarding another vodafone number other than the one you are calling from.