Tfillin comes from an Aramaic word, and phylactery from a Greek one. Jews have been speaking all kinds of languages since Hebrew stopped being spoken about 2500 years ago. (It was revived in the late 19th century.) They spoke Aramaic for quite a while (reason why a lot of the Talmud is in that language, and the Targum is entirely) and they also spoke a lot of Koine Greek (which is how the Septuagint came to be written in that language. Later they spoke a Romance language, from which Ladino, and in other parts of Europe a Germanic language, from which Yiddish. Not to mention German, Polish, Russian, Romanian, Hungarian, Arabic, Farsi, etc. As for which word English-speaking, observant Jews use, you could go to shul and find out, but I've noticed them using the word tfillin, though they usually know what a phylactery is.
