My roommate does this. He's from Pakistan and translates Urdu (I think that's how it's spelled).
At least with the place he works for, you only get paid for the time he's actually on the phone with people.
Some days he gets lots of calls, and sometimes not. Also, a call may be 2 minutes or 2 hours. His pay is something like $0.50/min (not sure exactly), but it can be hard to predict.
My sister did in-person medical interpreting for a hospital (Spanish-English). That seemed more regular.