I think the way it should be is that if you are the one giving the number, you say it once, and then they repeat it. I don't like people who insist on saying "ok" after every three digits. Also, when it comes to the last four digits, I like to just say them in one breath rather than pausing after the first two digits.
Email addresses? I don't like all that "M as in Mary" bullshit. It's very possible that it's just me, but that confuses me somehow. I'd rather just hear it once and then read it back.