SQL update with join

I need a simple sql that will update a missing date field from the single record (per call id) Call Header to the multiple record (per call id) Call Detail table.  
I've been trying:
UPDATE    calldtl left outer JOIN
                      callhdr ON calldtl.callid = callhdr.callid
SET              calldt.dtstamp = callhdr.dtstamp
WHERE     (DTStamp IS NULL)

but it won't take anything after the update calldtl - I tried inner and left outer and outer.
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