Nick, here are a few ideas to try when conducting research interviews by phone.

• Smile — even though they can’t see you. It changes tone, inflection, and attitude.
• Try investing more time at the start of a phone interview to build. It’s OK to start with some friendly chit chat before you ease into the meat of the interview.
• Start building rapport even before the interview. For example, make sure the tone of your recruiting and scheduling emails have the right tone.
• You may need to (slightly) exaggerate your audible reactions and encouragement on the phone. Don’t be creepy or inauthentic, but “mmm hmmm” more and laugh at their little jokes, etc. Up your charm! (Caffeine helps me here.)
• At the start, apologize and ask for participants’ patience with your non-native language skills. That might also give you license to ask more clarifying “dumb” questions.
• Experiment with some other recruiting techniques. Depending on the kinds of people you’re trying to reach, you might try (with carefully written screeners) things like Respondent.io, userinterviews.com, or Craigslist.