Hello
Thank you for your question and reaching out.
A "port" is a standardized channel on a router that allows you to receive traffic from other internet users. There are 65535 ports on a traditional router. Many ports are assigned for specific traffic protocols. For instance, HTTP traffic comes through port 80. SIP traffic comes through port 5060. RTP traffic varies between phone systems, but a typical range might be 10000-20000.
If your router or computer is using NAT (Network Address Translation) or a firewall, these features might close SIP and RTP ports so that packets never reach your phone. When the proper ports are not forwarded or opened, your calls could drop altogether or fail to initiate.
SIP is only a signaling protocol – it doesn’t actually carry the voice of a telephone conversation. To transfer voice between VoIP endpoints, SIP works in tandem with other protocols that transmit the voice information as payload. These include Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP), both of which are User Datagram Protocol (UDP)-based protocols. This means that SIP message exchange and voice packet exchange occur over two separate sessions, or channels.
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