I noticed this the other day, I am almost positive it doesnt charge but cant confirm. Step by step :
1. Dial 4444 (Top Up line, CX service number is 4445)
2. Press option 1 to top up your mobile
3. press Option 2 to top up by voucher
4. Enter a random 16 digit number or just hold on, the robot will ask you to input your voucher number again do the same.
5. The prompt will ask you to hold on and they will connect you with someone to assist you.
6. The advisor has no idea where the call has come from and assumes that you have dialled in using the official cfustomer servie number.
As I say i cant confirm that this didnt take any credit as I was using someone else's phone and hadn't checked the balance before calling, however i am 99.9% sure there is no charge.
Anyone able to test and confirm?