Hello kittybum66
We're really sorry to hear of the mobile service issues experienced by your sister and the inconvenience caused, we appreciate you taking the time to raise this via the forums and welcome to the community.
Do you know if the issues making calls is specific to one location or anywhere your sister has been? You mentioned she is able to receive calls, are the other services working such as sending and receiving texts and mobile data in these same locations?
We have help here for some mobile issues and you can check the coverage and masts in the area here, Please keep us up to date with this as we're eager to help.
Rob