NAT, idle timers, and split tunnels · Visual
One-way audio
Symptom
The agent hears the
caller. The caller hears
Phone
NAT
Count the NAT layers.
Two is too many.
Firewall
UDP
The idle timer is
compared with 30
Firewall
Split
Five9 names are not
excluded.
VPN
RFC1918
Private addresses are
translated.
VPN
RTP
SIP and RTP are both
allowed between the
Admin
Each hop has to pass media, not just the invite.
Sources and go deeper
Technical requirements PDF
.
vendor
. last checked 2026-10. Fetched for this lab in October 2026. Re-read before you buy hardware or open a firewall.
RFC 1918
.
rfc
. last checked 2026-10.
RFC 3261
.
rfc
. last checked 2026-10.
RFC 3550, RTP
.
rfc
. last checked 2026-10.
RFC 3551, RTP profile for audio and video
.
rfc
. last checked 2026-10.
ITU-T G.711
.
standard
. last checked 2026-10.
RFC 768, User Datagram Protocol
.
rfc
. last checked 2026-10.
RFC 4787, NAT behavior for UDP
.
rfc
. last checked 2026-10.