What are the requirements for becoming a language interpreter?

I've worked for companies where if I need a language interpreter, I call "the language line", and they find me an interpreter in any language I need. How do I become one of those interpreters? I have 2 native languages; spanish/english; knew both fluently since age 5. I also have bachelors degree in m communications; and I am a licensed P&C insurance agent. In addition; I am currently learning Italian.