Solomo Pro users, in particular, might find
SparRuf – mit dem Handy billig ins Ausland telefonieren interesting. Note that it offers "free" international calls from your mobile (if you have a rare flatrate that covers all networks). It's a callthrough service, and the number is 01570 248 0000 - a Vistream number. Which means that Solomo Pro users pay only 5 cents a minute to access it.