[igor.snezhko]

It will be superb, if 3CX will make some solution for small branches, that normally connected to 3CX Server at Head office directly, but loose their internet connection at some moment.

Currently only a few IP gateways (as I know) can to connect their FXO to FXS ports directly. This is Dlink DVG-7062 http://www.dlink.com.my/products/?idproduct=192 . It can provide this Site Survivability solution, but with tangled dialplan.

Moreover, I dont know any simple (and cheap) solution that can connect branch's IP Phones to branch's FXO gateway in case of 3CX Server connection falure. Maybe it can be some dialplan at each IP phone, but this not seems as simple solution.

Maybe, it worth to make a free 3CX SIP proxy with reduced functionality, that will connect all branch IP Phones to branch FXO gateway, so users can receive calls to this branch locally in case of link faliure?

Many IP phones and Gateways can connect to Secondary SIP Proxy, if a Primary SIP Proxy not ansfer for predefined time frame (all Grandstream units, at minimum).

It will be a great Site Survivability solution for cases with lot of very small branches. Again, I not mean I cases, where I can put 3CX Mini edition.

I speaking about very small and depended branches with 2-4 users only, that need some Site Survivability solution at the moment until internet link will be repaired.

[leejor]

A lot of this assumes that the remote site has it's own FXO trunks (for local PSTN calls), not VoIP trunks and/or not having all outside and inside calls passing through a 3CX located in a remote office. If the remote office has it's own FXO trunks then they are probably, or should be running their own 3CX at that location especially if internet reliability is a known problem. When it comes down to it, any company running mainly or exclusively VoIP trunking should be paying close attention to the reliability of it's service provider, not just the service in/to the remote offices.

[igor.snezhko]

The main idea here is that Remote Site don't need a full PBX functionality and relays on Main Office 3CX system.

It ONLY need some sort for Survivability solution in the event of Internet link failure.

Any mature system should take account of physical link failure.

Good example is Microsoft Outlook. The user can work with it emails, even if connection to the Exchange server temporary lost.

But in case of link failure to 3CX server, users HAVE NO CHANCE to ever receive (or place) calls from FXO lines. In emergency case they cant relay only on their mobile phones.

[igor.snezhko]

Yes, Grandstream also can do fail-over. But in very small branch (3-5 person) there is no reason to set up a separate PBX just for fail-over. The Gateway have to provide it automatically.

[salter]

How would you propose they implement this Igor?

Something like a computer locally running a small piece of call-switching software? If we're really trying to stay vendor-agnostic, one would almost HAVE to have some sort of 3cx-lite installed on a local server or computer that would route calls - and that would almost have to be the registration target for the phones all the time - else how can you ASSURE yourself that the phone will even know to switch over? I don't think all of the phone vendors support "Backup" registration targets.

The other option would be similar to Cisco's solution from several years back (they may still do it this way - I don't deal with their "big" voice platform very often) - they had a module within their multi-purpose routers that acted as a site survivability solution. Since those same modular routers were also the PSTN gateways, it worked well in theory. Can't say whether it worked well in practice.

Regardless - I love the idea of a survivability solution - its just a matter of figuring out how to get 3cx to do it right and stay true to their vendor-agnostic design.

[igor.snezhko]

Yes, this should be a 3CX Lite edition. It should have one predefined fixed group of Extensions. All external trunk should be always connected to this group.

To achieve redundancy you can use DNS system also.