Hans Van,

You are right as well. But some products require certain services to operate.

Erik explained it very clearly, I would add two more examples

1) A cell phone is of no use, if you don't subscribe to a service provider
2) A payphone is of no use, if the line has not Line Reversal or SMET.

Cadence solution is not universal as well. We have ,in Pakistan, many different switching networks like Siemens, Alcatel, NEC, Ericson and ZTE. Each of these has different frequency and cadence.

So Line Reversal is the best solution.