there are many more betamax clones that might use different access numbers.
In case that these accessnumbers are put on a list by o2 the only solution would be to setup your own callthru system.
You register a local german number for free at
www.sipgate.de
Take your dialnow account.
Open an account at pbxes.com and register the sipgate number as trunk and dialnow account as trunk.
In incoming routing you can choose callthru pin to activate your callthru. (don't know if it works on the free accounts, otherwise a soho account is available at 2,95 a month)
Call your own sip number, enter the pin, and you get a dialtone of the pbx and dial the number you want.