If he called back after that, they're definitely interested in you. By reaching out like this, he's indicated that he's willing to overlook the delay, mix-up, etc.
I got a job, after my email had buried the hiring manager's request for me to take a test.
So, you've got the explanation and the apology out there. So now don't really dwell on it--that will remind them too frequently of the problem, when it seems that they are ready to move on.
And this is good:
I think I would reverse the order. Apologize for missing his call, thank him for asking your contact to follow up and for returning your call. Then explain that the phone system wasn't working last week and his call may have been caught up in the mess while the system was being repaired.