DSL may not be the connection you're looking for
ADSL service over copper are not very reliable for a whole raft of reasons - underlying dial-up technology, copper wiring issues, interference and poor fault resolution times. A reasonable estimate of ADSL annual availability is around 99% - 3-4 day's outage a year. Single sites might go 15 months without an outage, but large estate of sites will experience faults with 1/15 of their sites each month.
Because of the dial-up nature of ADSL, most (80%) faults are cleared by rebooting the router - hence why ISPs tend to lead with that, rather than trying to wake up an engineer and get them to test the line or look at an exchange. Not every customer has tried it, even those that say they have.
Complex intermittent faults are always harder to diagnose and fix - and the customer's investment in their connection is often proportionate to the amount of support resource aimed to resolve the problem. Buy into an enhanced or superior care package and you can access engineers during the night and expect more attention, with quicker fix times and better compensation for failure to hit service restoration.
DSL is still relatively rubbish though.
Chosing a different connectivity mechanism can make things more reliable. EFM is based on multiple copper pairs - meaning a single faulty pair can degrade, rather than disconnect, the service. Business-grade EFM services can be had for a couple of grand a year.
If we assume that an ADSL line is 99% available, and an EFM line 99.7% available - and we combine that with an understanding of the cost of an outage (usually proportionate to revenue associated with the site and the IT assets involved), then we can generate an idea of the value of site that might justify chosing a more reliable connection.
We can do the same for a fibre-based Ethernet (~99.9% available, ~£8,000 a year rental for 10Mbps).
Many businesses have experienced the application creep where the Internet used to be limited to email and browsing, which is not often critical to the business. Increasingly Internet-facing business apps have appeared and been adopted without revisiting the connection and how valuable it is to the business.
I'll be interested to see the overall reliability characteristics of FTTC, because for me, this is more important to SMEs than capacity/throughput. Cabinets with single power feeds, lack of engineers trained in VDSL equipment, but less copper in the mix...