I had another problem how to use two SIM cards, one for calls and SMS, the other for Internet. I chose the settings like Tofra wrote, but everytime I wanted to call somebody, the data-card was chosen for calls.
Now I found the same feature to chose the card for making calls in my telefone-App.In the upper field (Name/Number/search) there is a field one the right side which offers aigain “settings”, and there you can chose again “call settings” and afterwards “other Phone account settings”. Now you see "Make calls with"and you can make the same choice which you can make in the SIMcard-settings under "preferred SIM for (Card1, Card2 or ask first).
That now solved my problem