The calls can be dropped because of the congestion too. You can check the NAP status for the congestions.
If you are receiving a congestion error,
You first need to check the concurrent call counts. If your concurrent call count exceded the license, you would have a congestion error. Please check your license.
Check if you set any Call Rate Limiting in the NAP. Go to NAPS → Select The NAP → Advanced Parameters → Call Rate Limiting
For other congestion problems please contact with support@telcobridges.com