If I were you I'd play it safe and call member accounting directly.
Just use the regular MS 800 number and when you get the prompts, touch for accounting. Have them tell you want fax number you should use, and any type of cover letter that should go on it.
If someone from these boards gives you a phone number and it's wrong, you could wind up faxing a copy of a check to someone else. Your check has a lot of personal information: Name, probably address, definitely bank routing number, and checking account number all printed on it.