[inquisitor]

The only prepaid SIM which undercuts the EU tariff for inbound roaming calls is simquadrat (a brand of sipgate). They don't charge inbound calls within the EU for 28 days after your last registration on the home network. What's also interesting is that simquadrat SIMs come only with a geographic number assigned by default, so you are even rechable at landline tariffs and if you want an additional mobile number you can easily add it in their customer portal within seconds.
The other downside besides the requirement to use the SIM in Germany every 28 days in order to keep inbound roaming calls free, is that you will need a German address to get the SIM delivered and you can only recharge it by bank transfer (or by automatic recharge through direct debit or credit card).
Another annoying fact is that despite simquadrat includes a VoIP service for in- and outbound calls they only terminate calls to the geographic number through VoIP because our anti-innovative regulator in Germany considers VoIP termination of calls to mobile numbers as unlawful as the higher mobile termination fees would no longer be justified if calls go through the Internet instead of an more expensive radio access network.