Cheques can by definition can never be obsolete as they are simply "a written order directing your bank to pay money [to someone]" and don't have to be on a bank issued and numbered format.
That's how I learned, red faced, to make sure my answers are something I'm comfortable reading over the phone.
If it's a written instruction, they do need to authenticate it, but they could call you to do so. Or they might charge a big fee for it.
Or they might drag their heels by claiming you have to perform the transaction online because of some policy they have that you don't care about.
But they definitely can't ignore a letter you send them.