Thanks! Sorry, I somehow missed your post.
Yes I wanted to set up an as reliable as practically possible sip service on fttc which was as functionally equivalent to a single POTS line with a single device plugged in as possible.
My primary issue is that the POTS line is over 4km long, and so has very high gain, resulting in an awful snr.
Now I am up and running I know a lot more about what the client needs are in practice, and I have manged to set that up. Turning off the voicemail did work in the end, and I managed to get a busy response after I set 'Supplementary Service Subscription' -> 'Call Waiting Serv:' to 'no' on my Cisco SPA112
Now all I need is the service reliability bit of my requirement to improve a good bit and I'll be happy. To be fair to the provider, I'll give them a bit more time on that one.
To be fair to the provider, though, their customer services attitude of 'why would you want to', especially when I managed to find a solution by myself makes it more likely that I wander off somewhere else in the future, though!