There are so many funny comments to make but I'll be good. LOL
Try this. Swap SIM cards. Turn off both phones and swap SIM cards. Reboot. Then try calling her and see if that works. If it does, then it's a setting on the phone that seems to be the culprit. If it doesn't work then the network for some reason is blocking it. OR if you have iPhones, sometimes if the phone number isn't in the contacts or sometimes even when it is, go into your settings and turn off the block unknown callers. That was a huge problem on mine and even blocked some of my contacts.