You should immediately take control of the call. You may start by introducing yourself, briefly explaining the nature of the call to the interpreter and requesting that the interpreter say a few words to the non-English speaker to ensure they can communicate. Next, you should begin speaking to the non-English speaker in the FIRST person, as if the interpreter was not there. For example:
CORRECT: "Hello, my name is John. How can I help you today?"
INCORRECT: "Tell her my name is John and ask her what I can do to help her today."
Remember to pause at the end of a thought to allow the interpreter to interpret.