if you have changed them in the last 30 days you can only add new numbers irregardless of your anniversary date, 30 days after you last changed them you can change them again
from your numbers t&c
http://www.o2-yournumbers.co.uk/020aYou can change Your O2 Numbers once every 30 days. You can do this by visiting o2.co.uk/youro2numbers, by texting SETUP to 21300, or by calling 2425 free from your O2 mobile (see point 8 for details).
If you don’t select your full entitlement of ten Your O2 Numbers, you will still be charged the full rate, but you can add to the empty fields by visiting o2.co.uk/youro2numbers or by calling 2425 free at any time.
