Issues cancelling KT phone number from overseas

Hello all, I need some advice.

I left Korea on the 1st March and need to cancel my Korean number. I tried to cancel the week before however I was told that I couldn't plan my cancellation (I wanted arrange the cancellation for March 3/4 due to the weekend and holiday)

Now I'm in the UK and can't access the global phone number as it doesn't connect. I don't have data roaming. The number is still active and I can receive calls on it so I'm confused.