ISDN/ADSL in Greece

Can anyone help me understand Greek ISDN and how to isolate/identify problem please?
I have a house in Greece with OTE CONN-X broadband over ISDN � the internet connection works fine but the voice channel(s) work only intermittently. I'm technically barely illiterate but the presentation is something like this:
OTE connect to external point on the house. From their termination, internal wiring to three outlets, only one of which is in regularly in use (although I have tried the provided equipment in all 3 with same result. Looks like only 2 wires presented (red/green) to the internal outlet. Straight-through RJ11 (I think) cable to a small un-powered box labelled LEA PCE 1381-21 rev.08 ISDN/ADSL splitter. One cable then goes to SAGEM F@ST 2444 OTE Conn-x ISDN router (then wirelessly to pc) and the other goes to an OTE-labelled SANTIS-ab S1608-K1200-A30-04 and then to telephone handset. The SANTIS box has 2 external indicator lights and one permanently shows orange � the other shows green when it's illuminated. If the green is illuminated the voice channel works fine but if it is not illuminated the voice channel only works sometimes. Both outgoing and incoming affected in same way.
Have had OTE engineer on-site and he says the problem is internal wiring and I need to find a local electrician to rewire � that's a problem on many levels here � finding an electrician at all in a remote village � then finding one who understands telephony � and then all the cabling is in trunking buried deep inside the walls and floor!
My suspicion is that the splitter is faulty but I've no way of knowing for sure and OTE reckon it's the wiring between their termination and the outlet.
Can anyone help explain how the internal wiring could be at fault if the ISDN connection (internet connection) works fine and the voice over ISDN intermittently doesn't? Seems they both use the same 2 wires so I can't get my untutored head round it.
Any help on how to isolate/identify problem without specialist equipment or ripping out the walls would be appreciated � in simple terms if possible.

A difficult problem to even make suggestions about.

Can I confirm that you actually have ADSL delivered over an ISDN connection.

The next stage would be some picture of the hardware, the sockets and the wiring behind them and maybe a drawing of how you believe it is all connected together. With the termination box/splitter can you get pictures of all the connections and sockets on that.

ADSL over ISDN confirmed - Sagem ADSL router

and diagrams etc here

http://s1264.photobucket.com/albums/jj497/cr31000/

Have you tried with a phone plugged directly into the splitter? Or with an extension cable back to where you need it? Than can rule out YOUR wiring.

First step is to remove the ADSL splitter and try the ISDN phones directly into the OTE wall socket.

If that works well, then add the splitter back but without the ADSL modem, if bad then, then you need a new splitter most likely.

If all is well with splitter add the ADSL back, and things goes noisy, this may still be a problem with the filter, or it may be a bad joint somewhere in the network, that is at this time only affecting the voice frequencies and not the ADSL ones.

Are they ISDN phones or are they standard PSTN devices?

Standard pstn phone plugged into santis box - either port same result

Another option is take the kit around to a friends who has same setup and see if it all works nicely there

Thanks for the help guys - have had to wait for the intermittent fault to harden - tried all suggestions and they helped confirm noise even on ISDN phone only. Decided to go back to basics and removed each phone socket in turn to check wiring. Last socket - in spare bedroom found had been inhabited by uninvited guest who had woven web/chrysalis or whatever round one of the connections - plus signs of dampness. Removed socket completely and now all ISDN/ADSL/voice channels work fine. Now just need new socket and will reconnect everything.
Many thanks for help
Cheers