Outgoing call rings once then disconnects! Can anyone help?

There is one number I am trying to ring from my mobile & it rings once & then disconnects. It’s an important phone call I need to make so I tried ringing it through my skype account & again it rings once but then comes up saying busy. I have rang this number in the past & it has worked. It’s an overseas call I am trying to make. I have tried other numbers & they connect as normal. Can anyone help? Many thanks x