I have stumbled over a few sites and most want to you get a special phone system along with a headset before you can work a job like this. Others will charge you a fee to work for their clients. If you are being hired by them, why is there a need for an extra charge?
They aren’t charging you a fee to work for them. In most cases they are charging you for a background check and not for the privilege of working for them. Also you aren’t being hired by them, you are usually hired as an independent contractor and you don’t work for them directly.