Using VOIP to bridge PSTN PBX?

Hello,
My company has three offices located in a small region. All three offices use a similar NEC PBX system, based on traditional switched circuit technology -- nothing fancy here. However, we are looking to implement VOIP to save on long distance calls between the offices, and also to allow (if possible) direct connection to individual extensions as opposed to calling the number of another office and reaching the secretary. Our users can call others in their own office simply by pushing a button on their phone and directly connecting to their extension. We would love to have this kind of functionality spanning all three offices.

My idea thus far (feel free to tell me if it is a bad one, I have very little voice experience) would be to setup an extension at each office that will, when dialed, connect the caller to a totally separate VOIP PBX that has only three extensions -- one for each office. After hitting this system, the user will enter a code or otherwise indicate which office they would like to be connected to, and then they are connected to the local PBX system and have the opportunity to enter an extension number for a local employee.

Does anyone know of existing software/hardware to accomplish this task? We do not have the capital to invest in a new VoIP PBX (as much as I would love it!) so we can't get new phones, etc. Is a setup like this possible with Asterisk?


Also, can anyone think of a more elegant/simplistic way of accomplishing my goal for the end users?

I appreciate any ideas you guys may have!

Provided you have some spare ports on your PBX the simplest and cheapest method would be to use a couple of Analogue Terminal Adaptors (preferably Linksys SPAs as they have full routing capability) as gateways.

Install one each end as as extensions off the PBX. You will then have the facility for direct dial to individual desks and full control of the line using the PBX

Thanks for your quick reply! I am very interested in your solution, and it sounds very workable, but I do have some questions about the implementation.

Firstly, what will the end user experience be like? For our inter-office telephony, users simply have to press a button for the relevant extension and are instantly connected to the person requested. Will this system allow all the users at the remote offices to be "buttons" on our PSTN phones, or will there be only one button for the Linksys extension, which will then route them to the remote office?

Also, since we will be connecting all three offices, will I need two SPAs at EACH office (TWO at office1, one connecting to Office2 and one connecting to Office3, and similar setups at the other offices) or will one in each location suffice?

Thanks for your help, it is greatly appreciated!

EDIT: Also, will the SPAs be able to manage the connections to eachother Adhoc, or will I need some sort of SIP server to pull it all together?

Sounds as if you have a key system rather than a full PBX? Which would limit things to the number of 'free' buttons on your phone. But yes, broadly it should be possible to set up button five and phones will ring in Timbuktoo, six rings in El Dorado etc. but you will not be able to pick an individual person's phone.

A full programmable exchange would allow you more choice as to dialing and would allow calling a specific extension.

For three offices you would, as you've observed, need two units in each office. You should also note that the SPA's only handle one call at a time.

This link might prove helpful http://forum.voxilla.com/linksys-sipura ... 19832.html