PSTN Pass-through

Here is what I was able to find on:

PSTN Pass through port:
What it can do:
- Local manual switching between PSTN and IP mode on a per call basis.
- User can switch to PSTN line by pressing *00 (or the configured
strings) for each call before they are placed. The device will revert
back to the default IP mode once the phone is hung up.
- It can allow a PSTN call to ring/call the phone connected to the FXS
- It also serves as a life line in case of power outage.
What it CANNOT do:
- Terminate a VoIP call into the PSTN port
- Allow a call from PSTN to route other VoIP devices (different from the
FXS phone) over the IP network
- Automatically route calls made by the local user to PSTN line

Note: On the HT-486 Rev 1.0, the PSTN port is only a life line port that
switches to PTSN only on loss of power.

FXO port:
It can support all the functions of a PSTN pass through plus:
- Terminate a VoIP call into the PSTN port
- Allow a PSTN call to call either the FXS phone or other VoIP devices
over the IP network
- Route call automatically and transparently to PSTN line according to
user configuration

> I just checked my dictionary and it defines "pass-thru" as meaning
> "totally useless for most people".  Pass-thru and lifeline seem to be
> different terms for the same thing.  i.e. The FXO port is connected to
> the FXS port in the event of a power outage, but other than that it is
> not useful.

Not quite.

A pure "life-line" FXO that is not voip accessable is useless to *.
Usually this means that an extension on the FXS port uses the PSTN on
the FXO during powerfailure / 911 calls. Some ATA's have this kind of

The SPA-3000's FXO CAN pass through in "life-line" mode automatically
for power faliures and if it is configured to do so via the dial-plan.
The dial plan on the 3000 allows lots of flexibility here. From a VoIP
standpoint, the FXS and FXO ports can be configured to be totally
separate devices, where if you want to make a call via the PSTN, the
call is looped through *.

"Pass through" can also be used in terms of how the FXO interfaces with
  • . The standard config of the SPA-3000 for example answers the call and
THEN forwards to * - acting more like a full gateway than a dumb FXO. It
can also be configured (kludged) to "pass through" call info to * BEFORE
the call is answered (which is frequently more desirable in many

Well, I didn't check any dictionary :-) but I've checked what other
people saying/posting about the AG-168 on their board at: "AG-168V Series"

From their conversation/questions it seems to me that that unit can
receive and make call to/from PSTN line.


Let me clarify this for you all. ATCOM's ATA does not have an FXO port.
The "Lifeline" port is not an FXO Port. It is an FXS Passthrough port.
It does not have any of the FXO features that you are looking for. You
cannot do a modprobe on this - nor can you pass your peer traffic to
this port.

Imagine this to be like an FXS Port with the Handset offhook and ready
for you to dial a number to call out using your existing analog line.
That's all it does.

This helps you make calls using your existing analog line in case of a
failure in your IP network.

Created by: jgabriels, Last modification: Mon 29 of Mar, 2010 (05:14 UTC) by spamblock
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