[asterisk-users] Why are 4 ports used for a single call?

[Bruce B]

Hi Everyone,

I am just tweaking a pfSense router and learning lots about NAT etc....I
noticed that each call uses four UDP port for RTP. Here is an example of
port for a call I made:

10200
10201
10504
10505

Seems like they are random in pair. I have a restriction of 10000-11000 in
my rtp.conf so that makes sense. But why use 4 ports per call? is that part
of SIP RFC?

Thanks
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