Looking for cheap ways to call Germany from the US (Washington, D.C.)

Hi there. My wife is German, and her mother, who lives in Cologne, is a bit of a luddite. In particular, she refuses to get Internet. This means Skyping, Google Hangouts, Facetime, etc. are off the table.

Are there any German expats here who have to make frequent calls to Germany on the cheap? My wife has an international calling plan with T-Mobile. Are there better options?

Vielen Dank!