[Asterisk-Users] How many FXS/FXO ports do I need?

[Rich Adamson]

> I'm new to phone systems and phone wiring and I couldn't find an answer
> to this question on the wiki or google.
> 
> My understanding is that a standard residential/business phone line
> carries the signal over 2 wires.  Your 4-wire RJ11 wiring supports 2
> phone lines.  Given that each line takes 2 wires, and there are 8 wires
> in an FXO port, can I conceivably support 4 phone lines on one FXO port?
> 
> On the phone/FXS side of things, can you also have multiple lines per
> FXS port?
> 
> If I want to hookup 5 phones to my residential phone service with 2
> lines, what # of FXO & FXS ports do I need?
> 
> Thanks for your clarification...

Others have already addressed most of your questions.

You have lots of different choices on how you address 5 phones and
2 lines. A couple choices include:

- use the digium TDM card with two fxo modules (to connect to the
  two pstn lines), and one fxs module (to ring all of the five phones)

- use two Sipura spa-3000 adapters (each adapter can support one pstn
  line and one fxs port, and you decide whether the two fxs ports
  provided on the adapters have one or more of the five phones
  attached to them. eg, business vs home phones.

- replace all of your analog phones with voip sip phones (and ethernet
  wiring). No need for fxs ports.

I kind of like the spa-3000 approach since those adapters allow
you to make a decision on how you want your system to function,
while also allowing you to change your mind and support your phones
in a different way at some later date. Lots of little features built 
into those boxes.

If you use the TDM card approach, any time your asterisk system is
down (for any reason), your phones are all down. That will likely be
a problem for you as you learn how to do things with asterisk, and
some of those "things" require you to stop asterisk & restart it.

If you use the spa-3000 approach, you can configure the boxes to
have all incoming pstn calls ring through to your phones (even when
asterisk is down, or AC power is down).