VoIP
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Tip
You can have a case of incoming calls working and not outgoing, which means registration has worked
but somehow you have incorrect proxy details. The other way around, where outgoing calls work and
incoming do not would mean the registration is not working, but the proxy details are correct. The
logging options can be very useful to help diagnose problems.
17.3. Home/office phone system
The FB2700 can be used as an office phone system (PABX) which allows you to connect a number of handsets
(telephones) in your office, and make and receive calls from the telephone number using a subscription to a
carrier over the Internet.
Traditionally a PABX would connect to one or more telephone lines, whether analogue or ISDN, and to a
number of telephone handsets. The PABX would allow internal calls (handset to handset) and external calls to
and from the external phone lines. It would have features like busy lamp fields and hunt groups.
The FB2700 does the same job as a traditional PABX, but using IP.
• Instead of phone lines or ISDN, the external calls are handled via an Internet Provider (ISP) and a VoIP
carrier. This can allow many calls and phone numbers to be used, and is generally a lot more scalable and
flexible than traditional phone lines.
• Instead of internal phone wiring to connect telephone handsets, the FB2700 connects to handsets via the
office LAN network. This can be the same network as used for PCs, or separate, or segregated using VLANs.
• Instead of analogue phones, or special system phones for a PABX, the FB2700 works with any standard SIP
VoIP phones. There are a wide range of phones available in a range of price brackets. The FireBrick has
been tested well with snom phones, including features such as busy lamp field lights and buttons.
• The FB2700 scales well to support hundreds of phones in an office without needing extra FireBrick hardware.
This makes the FireBrick a much more scaleable and economical PABX solution than traditional systems.
• It is possible to configure remote handsets, e.g. for home workers, connecting over the Internet.
The FB2700 can work with trunk carriers where one login/connection is used to carry many calls to different
numbers, or it can appear to the carrier(s) as multiple separate VoIP devices, or any combination. This allows
the FB2700 to be used with almost any VoIP carrier.
17.4. Network Address Translation
Network Addres Translation (NAT) is common on many Internet connections in homes and offices. It means
that the office uses private IP addresses (e.g. 192.168.1.1) and these are mapped (translated) to one external
address.
NAT is known to cause a lot of problems with a wide variety of applications and protocols. One of those that
suffers a lot from the problems of NAT is VoIP. There are, sometimes, ways of making this work, but it usually
a compromise of some sort and prone to problems.
The FB2700 provides some key ways to tackle the issues of NAT.
• An FB2500/FB2700 can be used as a gateway device in a home or office - using PPPoE to connect to the
Internet. This means the FireBrick has a real external IP address without NAT. The FireBrick can then
connect to SIP handsets on the LAN using private IP addresses. The FireBrick provides a gateway for VoIP
with no NAT implications.
Note
Some ISPs may use Carrier Grade NAT (CGNAT), and so a real IP address may involve additional
cost.