A plan type mismatch on the gateway results in dropped calls.
In order to fix the issue, make sure you send the correct plan type for National and International calls. To verify the call setup use the debug isdn q931 command.
Note: If you run debugs on the production router, it can degrade the performance of the router.
When you change the plan type to National, it fixes the problem.
Refer to these documents for more information:
Gateway or module
Call control software (CallManager, CallManager Express, ICS7750, SRST, SS7 call agents)
Call connection establishment
Dropped or disconnected calls
To international destination