There will be many “flavors” of SIP trunks.  Here are some examples:

- Public Internet – performance and security based on WAN connection – via hosted SIP trunks from a provider

- Private MPLS-Multi-Protocol Label Switching-VPN-Virtual Private Network used for customers with several sites and not visible for other customers via a private dedicated circuits

- Over IPSec VPNs for more security – requires key management, IPSec-IP Security implementation, real-time encryption & decryption, and potential multiple networks

- With TLS-Transport Layer Security and SRTP-Secure Real-time Transport Protocol used for highly secure situations.  This is where Port Level Security or TLS-Transport Layer Security takes place.  That is, control access by the Port Number such as deny or access to Port 5060-5061 for SIP or access to any other function.  This is called a Stateful Inspection for a firewall to check, alert or audit the status (state) of the TCP connection – SYN, SYN-ACK or FIN.

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