Bring us the denial
Tell us what the insurer said and when. A lawyer reads it before anyone calls you back.
For the second call
None of it is needed for the first call. It helps by the second.
The letter itself
The denial, the offer or the last email from the adjuster.
The policy
The declarations page at least, and the whole policy if you can find it.
Photos and video
Anything dated from before and after the loss.
Estimates and receipts
Contractor quotes, repair bills, hotel stays.