Alternatively you could record a mailbox announcement, in which you tell callers your temporary German number, and set an unconditional forwarding to your mailbox. I often do that when travelling, as it not only avoids roaming charges but it keeps away less urgent calls.
Regarding Fonic, they also have a referral program, through which you and another existing Fonic customer could get € 5 bonus credit, if you provide the existing Fonic customer's number during activation. E.g. VladS has a Fonic number, for which you could ask him.
You will find detailied information on availability, activation and recharging of Fonic at
http://prepaid-wireless-internet-acc...ermany+-+Fonic