Been there, done that.  I called Chase (the number on the letter matched the one one the website).  They couldn't help me and suggested I go to a local branch and talk to a branch manager (who would have more permissions.)  The branch manager could see that there was a credit card account open without an SSN on it but he was unable to get the SSN to stick.  He offered the help me fill out the W-9 and mail it to the credit card people.  I told him I wasn't going to bother, if they want to disable the credit card I've had for 27 years, I'd be happy to get a new one from a different bank.