Hi Marc87,
Thanks for your post and welcome to the forums. It's great having you on board with us in the Community.
I'm sorry to hear that you are having an issue with calling a specific number. As the call is failing and it's only happening with this number, it be worth the person who's number this is checking this. Do they have any other numbers that have trouble calling them?
Have they perhaps accidentally blocked the number? Is the number another mobile number, landline number or a premium rate number?
Let us know.
Thanks,