Certainly if you should charge or not for your conference call as well as how much you should charge is totally up to you. There are two different philosophies when it comes to charging. Some feel that they should offer conference calls free as a way to expose prospects to their services in a non-threatening way. Impressing potential clients with their business acumen or level of expertise in their field may work for you as an approach as well. This is certainly a very workable strategy as every one loves to get something for nothing. The other philosophy is that unless you charge a prospect will not assign value to your information.
When you don't charge for your conference call, you may unwittingly communicate that their attendance is not important to you. Attendance at your calls may suffer as the call appointment may be easy to blow off if there is a conflict or the day gets busy, because, you know it is free and there will most likely be another one.
Sometimes the best in between is to charge a modest amount and refund the fee against the first order or service contract. Whatever you choose, paid or non paid, you may want to test several options, check out your competition, and ask your own clients their opinions.