I would approach your situation in a variety of ways - depending on how you have your files set up.
1. If your WF file is managable, that's a good place to keep the bank letter until you receive a call back. This will also match your WF list.
2. If you have a tickler file, you could put it in today's folder - which someone else already mentioned. In this scenario, I would not put this item on a list unless I don't hear back the same day - then you need to decide if you should do #1, 2 or 3.
3. Since you most likely have a folder set up for this bank, you could put the letter in that folder; add the note to your WF list & reference where you put the letter so you don't forget.