If you log onto the online account manager, you can see your call information. There is no fee for incoming calls, no matter where they come from. They only deduct from your plan minutes. You will need to contact AT&T customer support with the phone information as to why you and your sister cannot connect. This is a user to user forum and that information cannot be answered here.
This is a user to user forum. I am a user, just like almost everyone else on the site.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.