Calling the customer to verify the signature was smart. Cashing the check knowing that is was not his signature was not smart; the bank should have just said "no."
It just set a precedent against which the bank's future behavior can be evaluated and it "trained" a teller that a signature unseen by the drawer can be ratified with a phone conversation for which there is no record. The better the phone call is documented, the more damning it is.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.