A company deploys a Cisco Unified Border Element (CUBE) to connect their on-premises CUCM cluster to a SIP service provider. After configuration, outbound calls connect and both parties hear ringback, but once the call is answered there is complete two-way silence. SIP signaling completes normally, and 'show voip rtp connections' on the CUBE shows local and remote RTP entries being created for each call. A packet capture on the service provider-facing interface shows outbound RTP packets leaving the CUBE, but no inbound RTP is ever received. What is the MOST likely cause?