Which SIM to receive calls while travelling and just keep number alive?
 

So I am not intended to do any calling from this number but it would be fine if it were cheap for roaming; receiving calls from abroad.

Also, do German SIM cards require a regular top-up to keep them alive or it is enough to place or receive some calls from time to time (like in the UK)?

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.

Thanks. Really useful post. Though I am not quite sure I understand the last part about call termination though I just looked up the term on Wikipedia.

Meanwhile, I also changed my mind. Maybe I'd like this solution better.

In telecommunications you look where a call comes from (where it originates from) and where it goes to (where it is terminated to).
simquadrat offers two alternatives of termination: One is routing the call to a SIM card in a mobile phone just as usual in mobile telephony. The other way, however, is to send the call over the Internet to a VoIP client, in other word the call is terminated to a VoIP client instead to the mobile phone.
In the latter case the call does not make use of the radio towers of any mobile network but is passed on to the VoIP infrastructure instead.
Now in most countries with the CPP (calling party pays) regime calls to mobile networks are charged at a higher rate than calls to fixed networks and these rates (called termination rates) are usually regulated. The national regulator regularly reviews the costs mobile network operators (MNO) charge to other operators in order to make sure the charged rate corresponds to the actual costs incurred for operating the mobile network.
Now if an operator takes an incoming call and sends it over the Internet to a VoIP client instead of using its expensive mobile network to deliver it to the subscriber, much lower costs incur as sending a few Kbit/s into the Internet is super cheap. Now this is where the German regulator says that it would be unfair to charge high mobile termination rates but then terminate the call into the Internet because the MNO would have a huge unjustified margin.
That's why simquadrat only terminates calls to the fixed number to VoIP clients but not those to the mobile number. Hence you cannot use VoIP to receive calls free of charge even abroad and without any temporal limitations.
I hope I was able to make this more clear now.

Interesting. I assume that might hamper O2 launching TuGo in Germany

Yet paradoxically if I get a German data SIM which supports VoIP I might receive calls on an O2 UK number free of charge while visiting there.