Hi Mihail.
if you want to filter your data before aggregation, please use WHERE keyword instead of HAVING.
Also unless you have any more complex metric, you do not need to use SUM around COUNT.
And finally - I am not familiar with your datamodel, but just as a reminder - the COUNT returns distinct number of values of its first parameter and takes optional second parameter - that one identifies dataset (table) in which you want to count it - in that case use the key of that dataset (I am not sure if Segment is key of the dataset where you want to count the Conversation).
So something like
SELECT COUNT(Conversation,[key of conversation dataset]) WHERE Communication Channel = Call AND Direction = Inboud
should hopefully make it work for you.