Digium SIP trunking comes with two plans.
Metered Rate Plan: Allows dynamically increase call capacity, meaning that multiple calls can be placed at the same time.
Channelized Rate Plan: Allows unlimited inbound and outbound call therefore there is no cost per minute, but limits the number of calls that they can place at the same time.
Most likely the customer has a channelized rate plan and he is exceeding the number of calls that he can place at the same, you will need to transfer the call to our sales team so they can upgrade their account.