Deloitte pre hire

Hi there, 

I've been offered a role at Deloitte and I'm due to start the beginning of March. They're at the pre hire stage, and I had to disclose I was fired from a role, (was an awful company in hindsight, and not one I had heard of. it turned into a call centre job, and it took on various staff and dismissed them before the 6-month mark so they didn't have to make them permanent. They claimed I had hung up on callers and hadn't reached the target the week previously which I believe to be untrue. Anyway since i've told HR this, i've not heard back and it's been nearly a week and a bit. Slightly worried, as i'm now wondering if this could jeopardise my role. They've also not asked for any previous employers details as yet.  I've handed my notice into my current work and I'm wondering now if I should have held off. Does anyone have any experience of working there?