Best VOIP App for iPhone on Hotel Wi-Fi

Hi, so my girlfriend is traveling for the next two weeks around Europe and the hotel she is at now has Wi-Fi but it's pretty crappy. I gave her a travel router to use but there is no Ehternet in her room. The Wi-Fi connection just seems like it is slow because she can use Viber to call me but it gets pretty choppy and compressed at times. Any ideas for a VOIP app that can handle this situation better?