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Would it be possible for you to try this with another SIM-card and subscription in your phone? It does sound like your number is being blocked by her phone.
If your operator allows it, you could also try calling her with a hidden number. To do that, go to Settings -> Call -> Additional settings -> Caller ID -> Hide number. Then try calling her and see if you can reach her.
If that doesn't work/help, would it be possible for you to borrow another SIM to test with or try your SIM in another phone?
If it's her phone that's blocking your number i'm afraid i don't know how to change that as i'm not that familiar with iOS. I found this article on Apples website though in case it helps: https://support.apple.com/en-us/HT201229