Sophos Firewall supports VoIP using both Session Initiation Protocol (SIP) and H.323 standards.
Your VoIP provider will supply the configuration details for your VoIP system. You'll then need to configure a firewall rule to allow either the SIP or H.323 service.
The H.323 and SIP standards provide a foundation for audio, video, and data communications across IP-based networks. These standards allow you to participate in video and audio conferences, even though you're using different applications. H.323 and SIP use different flows for signaling and data transfer. Signaling flow is used to negotiate the configuration parameters, IP address, and port to establish the data flow. These protocols are harder to filter by firewalls since they violate layering by introducing layer 3 and 4 parameters in layer 7 of the OSI model. To overcome this situation in iptables, Netfilter provides connection tracking helpers, which are modules that assist the firewall in tracking these protocols.
The most common issues encountered with VoIP are poor call quality, one-way audio, or calls dropping. To resolve some common issues with VoIP, see the links in the following section.