You could try something like this:
First Formula - we'll call this "GetMinTime"
NumberVar MinTime;
if (previousIsNull({call_number})) or
({table.call_number} <> previous({call_number}) or
(MinTime > {seconds to respond}) then
MinTime := {seconds to respond}
else
MinTime := MinTime;
MinTime
Second Formula - we'll call this "MinTimesTotal"
EvaluateAfter
{@GetMinTime};
NumberVar TimeTotal;
if previousIsNull({call number}) then TimeTotal := 0;
TimeTotal := TimeTotal +
{@GetMinTime};
TimeTotal
Your average then becomes something like this:
I haven't tested any of this, but hopefully it will point you in the right direction.
-Dell