[asterisk-users] how many UDP ports is required for 1 call

[Gilles]

On Wed, 22 Feb 2012 18:04:38 -0600, "Kevin P. Fleming"
<kpfleming at digium.com> wrote:
>SIP is most commonly transported over UDP, not TCP.

To complicate things further, there's "symmetric RTP":

"A device supports symmetric RTP if it selects, communicates, and uses
IP addresses and port numbers such that, when receiving a
bidirectional RTP media stream on UDP port "A" and IP address "a", it
also transmits RTP media for that stream from the same source UDP port
"A" and IP address "a".  That is, it uses the same UDP port to
transmit and receive one RTP stream.

A device that doesn't support symmetric RTP would transmit RTP from a
different port, or from a different IP address, than the port and IP
address used to receive RTP for that bidirectional media steam."
www.armware.dk/RFC/rfc/rfc4961.html