[asterisk-users] VOIP with PSTN backup

[Brian Candler]

I'm looking for a way to set up a VOIP network in branch offices where one
or more phones have "lifeline" capability, i.e. can place calls if the IP
network or VOIP service dies, or even if power goes down. (I'm thinking of
business continuity here, not just emergency services)

This seems to limit my choice of products somewhat, and I was wondering if
anyone had recommendations for use in this scenario.

The approaches I'm thinking of are:

(1) Use an ATA with PSTN passthrough or FXO port, and connect an old
analogue telephone to the FXS port.

In this case, the analogue phone has lifeline. If there's a true FXO port
then PSTN calls can in principle be routed to/from other VOIP phones in the
office (but see below)

There seem to be a few to choose from, although far fewer with a true FXO
port.

(2) Find a VOIP phone with integrated PSTN or FXO port

In this case, the only one I have found so far by searching the web is
Clipcomm CP101.

I have also read that many FXO devices tend to be badly implemented; in
particular, on seeing ringing voltage, they actually pick up and answer the
call, instead of sending off a SIP INVITE and waiting for the OK before
connecting. I'd certainly like the device to behave properly in this regard.

As a second part of this question, it would be extremely desirable if the
backup PSTN service were available to all the phones in the office. That
means:

(a) incoming PSTN calls could ring *all* the VOIP phones in the office, not
just the one phone or ATA connected to the PSTN line; and

(b) any VOIP phone could route a call out over the LAN to the local FXO PSTN
port, e.g. by dialling a prefix to access it.

This isn't so essential but it's definitely desirable. Any recommendations
for how to do this too?

A large number of offices is going to be involved, and I want to keep as
much switching "intelligence" centralised as possible, both for ease of
management and to keep the cost down. That is, I don't want to install a
PC + TMD400P + Asterisk in each location, but just a small media gateway or
VOIP phone.

However I can see that the incoming ringing issue will require call forking,
so I am happy to install an OpenWrt box running Asterisk or siproxd or
whatever in each site. Being diskless and low power should mean little
maintenance is required. But such a box isn't going to be able to take an
FXS/FXO card, so I'll still rely on an ATA or VOIP phone to present a PSTN
interface. So that's the key part I'm looking for.

Finally, the devices must be robust (i.e. not need power cycling every 24
hours) and centrally manageable.

I think that's about it - many thanks for your ideas and experience!

Cheers,

Brian.