Be about to is used to talk about things which are going to happen very soon:
I’m about to eat. Can I phone you back?
It is often used with just:
We’re just about to set off for a walk. Do you want to come?
When used in the past, be about to can refer to things that were going to happen but didn’t:
I was about to complain but he came over and apologised.
We don’t use be about to with time expressions:
I was about to call you.
Not:
I was about to call you in ten minutes.