Gigaset on 2Talk and Spark?

I am a Spark customer and my plan gives me free national calls. I am also a 2 Talk customer. How can I set up my Gigaset phone to make local calls and national calls with Spark ( which are free) and other calls like international and mobile calls which I pay for with 2 Talk? The reason I am keeping my Spark number is to keep my listing in the White Pages