What’s going on?
Can you tell me what’s happening?
Who’s in charge of this?
Could you look into this?

Responding to complaints

I’ll look into this and get back to you.
My colleague Mr./Ms X deals with this. I’ll get him/her to call you back.
I’m afraid there’s been a problem with…
I’m sorry about this. We’ve changed our procedures so it won’t happen again.
We apologise for the mistake. It won’t happen again.
Our apologies for the mix-up. We’ve sorted things out now.
We’ve looked into your complaint and it seems to be justified. Can we offer you some sort of compensation?