CASE with 2 conditions
Not sure if only that, but your are not closing the parenthesis correctly.

Same code with better indentation:

case
  when
    (product_name = 'premium' AND SUM(answered_calls)&gt;0
  then
    (SUM(total_duration)/60.0)/SUM(answered_calls))
  else
    0
end AS Duration


See the problems:

case
  when
    (product_name = 'premium' AND SUM(answered_calls)&gt;0
--  ^----------------------------- should close here --^
  then
    (SUM(total_duration)/60.0)/SUM(answered_calls))
--                      remove that closing one --^
  else
    0
end AS Duration