[inquisitor]

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.