FXO FXS cards

i am looking to implement a freepbx in to my bosses business that will not cost him a arm and a leg. I have done most of the research already and i have the freepbx distro up and running on a machine.
I know what i have to get at least a FXO FXS pci card, but what i will like to know or if anyone can tell me is how many extensions can i get from a FXO FXS card.
Like say 1 FXO port and 3 FXS port or a pci card with 4 FXS ports, or vice versa. Can i double up on one FXS or FXO port using a RJ11 adapter to get 2 lines out of one port. And will i be able to configure that one port that has the adapter with 2 different extension.

It is a one-to-one relationship. You need an FXO port for each phone line.

I mistyped. Correct. FXO to Analog Phone lines provided by PSTN operators and FXS to Analog devices (like analog phones, FAX G.3 devices and so on). The rest should be quite OK inverting FXO with FXS and vice-versa. Thanks (I’m not able to correct the post once submitted).

Why do you want to use FXS ports? Also FXO ports are for analogue phone lines. It might be better to go to a VoIP service provider and get the existing number(s) ported to that new service provider. It’s a lot easier and more flexible that using old analogue line technology.

IMHO you’re confusing what is a PCI card (or Cards, if many are going to be used on a system) used to connect to Trunk lines (usually phone companies provide “lines”: analog, commonly called POTS, or digital, commonly called ISDN BRI/PRI).

Trunk lines are used by your VoIP system (in this case FreePBX Distro, as example) to connect with the “outside” world (I mean “outside” in telephony jargon) in comparison to internal extensions (your desktop/cordless IP phones, as example) that shouldn’t be called lines.

In a VoIP system you can have various Trunks to connect to “outside” world (protocols, settings and services are very country specific):

  • Analog lines -> so you need one or more FXS PCI card with a number of ports to match the number of physical analog lines your phone company is providing you.
  • Digital lines -> so you need one or more ISDN PCI card (number of ports: same as above).
  • SIP Trunking -> you don’t need any PCI cards because your VoIP system can register (obtaining one or more geographical telephone numbers) directly through your Internet connection to any reasonable number of SIP Trunks (depending on the Bandwidth and QoS of your Internet connection!) provided by a VoIP SIP Trunking provider.

In this latest case (and ISDN too) the term “line” could led to confusion because in the case of Analog lines you have 1 line = 1 number and 1 voice bi-directional channel, in all other cases things changes: ISDN BRI could have 2 voice bi-directional channels (up to 30 with ISDN PRI) and many telephone numbers associated with them (from 1 to many). In the case of SIP Trunking the concept of 1 line = 1 channel loses its sense.

Or any valid combination of those Trunks configuration listed above.

Internally you have Extensions: VoIP phones that shall support SIP…so basically you need only a good LAN cabling (each VoIP terminal acts like a workstation and can provide a Fast/Gigabit-Ethernet mini-switch that let you use only one LAN cable to the desktop to serve both the VoIP phone and Workstation).

It’s very unusual to implement a VoIP system (I’m speaking generically) to serve interannly analog desktop phones: only in this case you’ll need a lot of ATAs (SIP/Analog Media-Conveters) or a lot of FXO PCI Cards each one with a proper number of ports equal to the number of Analog phones you want to deploy. A real nightmare!

This is just incorrect. FXS ports are used to plug analogue devices (phones, faxes, modems, etc) into whilst FXO ports connect to phones lines. You have it the wrong way round.

We have a wiki article outlining telephony options, that covers FXS and FXO.