Thanks for joining our Support Community to find out why you're being charged when calling your company's WebEx conference number. I believe that specific number may be considered an out-of-plan number. You can find more info about that here: Out-of-plan phone numbers. Please let me know what other questions you have after looking over the link I sent.
Amanda left some really helpful info. Have you had the chance to take a peek at her post? Please let us know. Thanks!