HSBC reinstated my account after I paid it off. I went through the regular customer service phone number.
My account was never charged off, but it was sent to collections and was 90 days late. They closed the account and sent it to a collection agency. I scraped together enough money to make a fairly large payment that covered all of the past-due payments and fees and got the account under my credit limit. Then I just kept making monthly payments through the website like I did when the account was open.
After a year of on-time payments I called and asked them about re-instating the account. They said they couldn't consider reinstatement until the balance was paid in full. At that time, my balance was around $500.
So when I got my income tax refund, I paid the full balance. I waited a week and then called again to ask about reinstatement. The customer service rep said they would put in a request, that reinstatement was not guaranteed, and that they would pull my credit report (a hard inquiry). She told me to check back in 10 days.
When I checked back, the account was reinstated. It's now my oldest tradeline and in a couple of months my late payments will all be more than two years old.