Thank you for replies.
some clarifications for the last post:
Calls are not from EWSD. All the calls are originating from the E1’s and are send to EWSD. Operator claims that our calls are causing congestions of its switches and morover result in congestions of voice trunks between the switches. Most of our calls are to mobiles (operated by the same operator) and operator claims that these calls overload also the trunks laying between the EWSD switch (we are connected to) and the switch handling the GSM mobile network.
For a network based on 4 or 5 core switches, passing 800 million minutes a month, it sounds to me very unrealistic, that only 120 simultaneous calls may overload the links between two of those core switches of the operator network.