JOIN Call table to grouping by Origination and Destination
I think you need to change one join column on the inner join from tbl_calls to  tbl_origination_clients. You have currently c.des_client_id but it should be c.orig_client_id.

Query with adapted join column

select
  co.country Origination,
  cd.country Destination,
  sum(c.call_duration) TotalDuration
from tbl_calls c 
inner join tbl_destination_clients d on d.des_client_id = c.des_client_id
inner join tbl_origination_clients o on o.orig_client_id = c.orig_client_id
inner join tbl_countries cd on cd.country_id = d.country_id
inner join tbl_countries co on co.country_id = o.country_id
group by   
  co.country,
  cd.country;


Result

| Origination | Destination | TotalDuration |
| ----------- | ----------- | ------------- |
| Austria     | Denmark     | 9             |
| France      | Brazil      | 11            |
| France      | Lebanon     | 10            |


DB-Fiddle