Which card for my purpose?

Hi all,

I’ve never configured Asterisk to work with analog lines, I’ve always configured PRI lines.
I have an Asterisk server which act as an IVR. It will be connected to another PBX trough a digital to analog converter, like this:

TELCO -------> OTHER PBX ------> digital to analog converter -------> IVR

Now, the IVR should be able to receive 4 simultaneous calls coming from outside and it should also be able to place 4 outgoing calls to the external line.
In addition, it should be able to receive faxes via 2 dedicated lines and redirect them to a Hylafax server.

My problem is that I’m a little bit confused on FXO and FXS.
Which card should I purchase? And with which configuration of FXO/FXS ports?

Many thanks



If you’re connecting Asterisk to the extensions on the other PBX, then you’ll want FXO on the Asterisk side.

You need support for 10 simultaneous channels?

Thanks Malcom for your reply.

Yeah, that’s what I need. 10 channels.
I supposed FXO was what I needed but now I have the confirm.
My doubt was that FXO could not place outgoing calls to another pbx.


FXS and FXO are just different sides to the interface. You receive calls across FXO (the other side is FXS) and you place calls through FXO (the other side is FXS).

If you need 10 simultaneous calls, you’ll need to dedicate 10 FXS interfaces on your other PBX and you’ll need 10 FXO interfaces on the Asterisk side. You can terminate 10 FXO’s into Asterisk using this (www1.digium.com/en/products/tele … og/24-port) modular card. The card can be populated by up to 24 ports, you only need 10. The modules come 4-channels per module, so you’d use 3 of the FXO modules on that card. That’d be the TDM2403 (PCI) or AEX2403 (PCI-Express).

You can also use an IP-FXO gateway instaed of a card.

Thanks Malcom,

I’ve understood how FXO/FXS work.
I thought my solution was to purchase 2 Digium analog cards, one with 8 FXO ports and another one with 2 ports dedicated only to fax receiving.

What do you mean for IP-FXO gateway?

It converts analog signal from analog lines into sip (and vice versa), so you can register it as a SIP peer on asterisk. You can check for D-Link or Grandstream SIP-FXO gateways.

You can do either - one of the 24 port cards (loaded with 3 modules )or one 8 port card (fully loaded) and one 4 port card (loaded with only 2 modules)