Essentials for a pure PSTN routing & Asterisk's capabilities

An absolute newbie here, forgive me for being so naive, I have a question for this vibrant community of asterisk pros a simple question.

We have a UAN in our main office with 4 PSTN lines. We have two other offices with one PSTN lines each.

What we want:
We want to setup an exchange solution where we plug in 4 PSTN in our main office to the exchange solution (using some hardware if Asterisk has any). and receive those calls and our exchange setup should present caller with three options (press 1 for our main office, 2 and 3 for either of our remote offices).

If caller doesn’t dial a digit, or dials 1, it should connect the caller with the head office operator and if needed that operator should be able to forward their call to 2 & 3.

If 2 or 3 is pressed, (either by the customer or our operator ), exchange should be able to forward/dial/direct our customer to the relevant office’s pstn line (im not sure if this is possible using asterisk or not). we can have separate pstn lines for dialing our office numbers and somehow connect customer though exchange to our other office of if can can forward their calls to the direct pstn lines of our other offices, that too would work…

my question: Is it do able using asterisk (with any hardware they might offer), and if it is, is there a resource for us to find information about this particular setup (or something similar).

A raw idea of what i had in my mind, please let me know if its conceptually correct and possible and if asterisk can help us achieve that or not.

Thank You!!!

One of the best sources of information about Asterisk is the official Wiki.

There is Also a book ‘Asterisk the Definitive Guide’

You should be able to do exactly what you want with Asterisk and either a interface card or a media gateway to connec to the PSTN.

Support for actual forwarding is weak for analogue lines and not available for non-VoIP digital lines, so you will need to relay the calls through Asterisk.

Analogue lines, as I think you are assuming, have difficulty with supervision. You should confirm that the PSTN operator provides disconnect supervision that is acceptable to Asterisk, otherwise Asterisk may never release the outgoing line.

Generally, if you want to do anything like this, you need ISDN, or VoIP, rather than analogue lines.

I’m not sure what you want is really possible using the PSTN lines. Your primary challenge as I see it would be device states for one (difficult over analog connections), and limited abilities to place outbound calls (you need dedicated lines for those remote offices).

If each location has Internet, you are far better off choosing a central location for your Asterisk server (main office) and just putting Endpoints (Internet connected Desk Phones) at the remote offices.

You can do what you want with just an analog setup, but it will be just as expensive as the VoIP hardware, and less supportive of features requiring device states (automatic call distributions/queues). For instance, each one of those six PSTN lines will require a FXO/FXS port. You need 4 FXS, and 2 FXO ports. Not to mention you’re paying hefty usage fees too. Those incoming calls are double usage if you need to forward it to a remote office.

I’m assuming you have 4 phone numbers (DIDS) you wish to keep? Port them to a reputable provider (I use Flowroute, but there are others like Twilio, Voxbeam) and then use VoIP to send that traffic to your Asterisk server in your main office. The benefit with that will be the ability to accept 6 incoming calls at once, instead of 4. You have the ability to for each Endpoint to carry multiple outbound conversations simultaneously.

There are quite a few drawbacks to insisting on analog support. If you can ditch the analog lines, it becomes much easier and cheaper to support features like IVR that your current system is performing.