A company uses Expressway-C and Expressway-E for business-to-business (B2B) video calls. Outbound calls to a partner domain (partner.example.com) fail immediately, while inbound calls from that partner succeed. From the Expressway-C, an administrator can ping the partner's Expressway-E by IP address. A packet capture shows no SIP INVITE ever leaves the local network for outbound attempts. What is the MOST likely cause of the outbound call failure?