FaceBook Messenger?

tawk.to was very easy to set up.

Depends on what your requirements are.

Some systems require the operator(s) to run a program on their computer. Others offer a web interface for operators. Some offer a mobile app you can run on a phone.

Some only allow one operator, others allow more than one but you pay extra depending on how many operators you want.

Some allow an operator to handle just one caller at a time, others allow several calls to be handles by a single operator.

Some allow the caller to send files to the operator, others don't.