There are many reasons why calls are failed. Sometimes, it can be calls that are dropped by the switch. Sometimes, it can be calls that are blocked by the vendor.
Here are some steps to go about solving them.
You can click on the "Counts" to look into CDRs:
These are the various errors that your vendors are returning for all your calls sent to them. Note that Class 4 may attempt multiple vendors for each inbound calls. Therefore, you will see larger count value in Vendor side than the Client side normally.