NEC SL1100 Networking Manual page 180

Issue 6.0
9 - 46
In the original setup message there is a field labeled SDP (Session

Description Protocol). The SDP portion informs the IP phone where to
send the media (voice) to. The SDP portion of this invite message
contains the IP address of 192.168.2.15 (ext 120).
192.168.3.26 (ext 130) sends a 200 OK message to 192.168.1.10

(VoIPDB). In the 200 OK message is the SDP field reporting the IP
address of 192.168.3.26 (ext 130).
192.168.1.10 (VoIPDB) forwards this message to 192.168.2.15 (ext

120).
192.168.2.15 (ext 120) sends an ACK message to 192.168.1.10

(VoIPDB).
192.168.1.10 (VoIPDB) forwards this message to 192.168.3.26 (130).

At that point the two IP phones attempt to send voice packets directly to

each other. As there is no VPN connection between these sites the call
is set up with no voice path.
To correct this issue another VPN connection between sites B and C is
required. If an additional VPN cannot be implemented, due to network
limitations, the Peer-to-Peer feature can be disabled in the NEC SL1100. With
Peer-to-Peer disabled, all packets (Signaling and Voice) route through the
VoIPDB card. This also affects IP phones at the REMOTE locations calling
other IP phones at the same location. Without Peer-to-Peer enabled the voice
path must route to the MAIN location and then back to the REMOTE instead of
directly between the two stations on the REMOTE network. This forces the
use of additional bandwidth on the MAIN, and REMOTE locations.
Peer-to-Peer is disabled in command 10-26-04.
NEC SL1100
IP Multiline Station (SIP)