Always "face to face" over "phone". Interview is about communication, which is much more than spoken language. You need to sense your interviewer. Plus, the noise etc associated with a phone call can really affect non-native speakers, especially if you aren't too comfortable with English to begin with.
i second to tuxiaoyang's post. when i inverviewed a job candidate on the phone last year, he nearly put me to sleep.... (i guess i share the blame too. :p )