Check vPanel and see if there's an entry in your call log for one of the problem calls. If so, click on either the From or To number and it will show you a Call Reference ID. Open a support ticket explaining what's happening and include the Call Reference ID in the ticket. VOIPo engineers can use this number to track how the call was routed through the network. It may be helpful to also include a Call Reference ID for a call that worked without issue so they can compare the two.