A NAT ALG is similar to a firewall ALG, but a NAT ALG actually changes (maps) the addresses and ports in the signaling messages. The NAT ALG cannot inspect the contents of encrypted signaling messages.
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab11/collab11/security.html
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X14-
0/basic_config/exwy_b_cisco-expressway-e-and-expressway-c-basic-configuration-deployment- guide-x14-0/exwy_m_advanced-networking-deployments.html