SPA-3000: PSTN problem. Possible hardware fault?

I have never really used the FXO port on my SPA-3000.
However, I thought I would try coupling up a Fritzbox FXS line to the
FXO on the SPA-3000.
I can see 43V is present on the web info. page, for the PSTN line, but
there is no activity of any kind in or out from the FXO to the
handset, i.e. no ringing, no dialling out is possible etc.

I tried unplugging the power supply from the SPA-3000 and
communication worked OK both ways - i.e. the phone would ring and the
incoming call was OK and I could make an outgoing call.
The PSTN line is enabled and I can't see anything wrong in the
settings.
I ran a simple test with the dial plan :-
<#00:a test number><gw0>
I am beginning to think that there might be a hardware problem.
Has anyone any suggestions?

 

Is the SPA line polarity sensitive?
Is the impedence set correctly at both ends?
Is the ringing cadence from the fritz what the spa is set to expect?

As for calling out - is the dialtone from Fritz exactly what the SPA is
expecting? possibly tell the SPA to blind-dial [ignore dialtone] and see if
that works.

- put a capacitor in series with an earpiece and listen to see if dialtone
and/or ringing are actually on the line when you expect them.
- check that when the SPA picks up the line the DC drops to well below 43v
to signal dialtone generation wanted.

 

Thanks for your suggestions.
In the end, after baqcking up my settings, I did a full factory reset
and checked it out from there.
The upshot was just one setting that I had ignored and this affected
both incoming and outgoing calls.
It was that 'Use Pref Codec Only', under tha PSTN tab, was set to
'Yes'. Once I changed that to 'No' it worked!