In case anyone is following this thread and wants to set up something similar, I will tie up some loose ends here.
The setup described in Reply #4 works. Some of the settings have been changed to suit UK PSTN conditions, so just leave them at default if you live in North America. One disadvantage of the way I have set this up is that I have used up a complete spX slot. This was not a problem in this particular case and simplifies many other settings regarding routing calls both ways between PSTN and the softphone on the laptop.
A particular problem in this case regarded CPC (Called Party Control). CPC is a short drop in line voltage sent from the callers equipment back to the OBi to say they have gone on hook. In the UK this is typically between 90ms and 110ms. Either the land line or the wiring in the home in this case seemed to simulate a CPC signal as soon as the PSTN line was seized. This is probably due to a line reversal combined with some capacitance making the OBi see a short drop in voltage. This would not be noticed by a normal phone. This lead to all calls dropping out as soon as the PSTN line was seized. The solution was to disable CPC and rely instead on DetectDisconnectTone.
There remains some fine tuning to get call quality up to a good standard. This a mix of factors relating to using a laptop (speakers and mic close together). A headset is not a suitable answer in this case.
I was going to float the idea of trying other softphones that need to register to call and get them to register to the OBiON App for PC, but that app seems to have disappeared for some reason. The SIP signalling would have bounced around the world, but the speech (RTP) would have connected locally within the users local router subnet, so it would not have introduced any lag etc.
