Thank you for your replies!
For Tony F:
The operator currently abandoned the idea that the EWSD switches are being congested and they moved to the version where the trunks between one of the core EWSD switches and the GSM netowrk are overloaded, exactly as you explain (note that the GSM netowrk is also owned by the operator).
However I would like to provide some figures about the transmission capacities of this operator:
The operator has its own fiber optic cable over 7000 km in length and more than 200 POPs based on SHD/DWDM technology providing nation-wide coverage.
City-rings of three major cities have actual capacity of more than 200 Gbps.