How about the charges? 费用如何？

It depends. 要视情况而定。

The charges vary according to the types of call you make. 费用根据您打电话的种类不同而有差别。

What about the changes? 费用怎么算？

The charges vary according to the types of call you make. 费用根据您打电话的种类不同而有差别。