Patrick - welcome to the forum.
The general principle for the RJ11 is that the two wire line ("tip & ring" or "a leg and b leg") always uses the middle two pins. This is normally pins 3 and 4. If there's a second line it will use pins 2 and 5. The OBi110 has only one phone line and one PSTN line so they both use pins 3 and 4 at the OBi end. It looks like your phone cable can be a straight RJ11 to RJ11 cable.
The British Telecom style 431A socket uses pins 2 and 5 for the two wire line. If your PSTN line is coming in from a British Telecom socket, then you need a connector or cable that connects pins 2 and 5 of the UK style 431A wall socket to pins 3 and 4 of the US style RJ11 plug that connects into the OBi. Adaptors and cables are easy to find on Amazon or Maplins.
Ringing voltages are no problem. However, by default the OBi110 is set up for North American defaults. As you bought it on Ebay, then I guess it was set up however the previous owner wanted it! It may be wise to do a factory reset first to get back to default settings. I suggest these changes first:
I suggest setting the line port impedance to 370+(620||310nF).
For the phone port impedance I would probably select 320+(1050||230nF) which looks closest to BT's nominal values.
Both Line and Phone ports should be set to FSK(V.23) method for CallerID.
Line Port RingDelay can be set to 0.
Have a look around the forum for UK dial plans / digit maps. Getting OBi set up to match the UK numbering formats will speed up dialling a lot.
Post back if you have other UK questions. There are a few of us Brits here on this forum
