How many people have ISDN lines, we have multiple lines in our office and not a single ISDN line!![/quote}
Anybody using a PBX supporting multiple extensions will normally be using ISDN circuits. Anybody just needing a separate fax machine and telephone would normally rent ISDN 2. Generally, people renting multiple single lines have grown slowly and added new ones as they need them and have expensive and extremely inflexible telephony.
As for independant ADSL's being foolproof, i beg to differ - the one and only problem we have with our ADSL service is the exchange falling over, its never the service provider so having 2 of them wont make it better, it will just double the cost and not solve the problem.
Nothing is foolproof. ADSL IS less reliable that PSTN - the ONLY advantage of the legacy PSTN is that it's been around long enough to work most of the time. Having two of them DOES make a big difference, particularly if you use one ISPs that has their own equipment on BT's exchanges.
But I don't recommend the use of two DSLs just for failover - it's to separate calls from internet use and is only necessary if you need several simultaneous calls whilst heavily using the internet for heavy web browsing and internet.
For normal use a single DSL is fine.
Suggesting you "unplug the phones and go home" is a silly idea, people are running professional businesses, if they could simply do that without other consiquences then they probably wouldnt have an office in the first place!
I run a telephone company with tens of thousands of customers and don't have an office - often it's only habit that makes us think we need one. But of course many/most people need one - even so the ability to have your office number ring in several other places is extremely useful, it means you can sometimes work from home if it suits - you don't actually need to take your phone home of course, you just have another one there.
The fact that if necessary, you COULD run your business from another location in an emergency without cost or difficulty is a massive advantage and something that is utterly impossible with legacy PSTN