Encourages keeping you on hold...
When a company uses a kickback number like this, they gain a financial incentive for keeping you on hold as long as possible. Some companies will actually work out the optimal length of time to keep someone on hold, balancing the revenue they make against the irritation of the customer.
Conversely, it's very rare to be kept on hold when calling an 0800 number, as the cost is reversed.
I don't think companies should be able to profit from inbound calls, unless they are offering a specific service directly related to the line (eg dialup access or chat lines etc)... And ALL such services should be regulated just like the higher rate 09xx numbers are currently.
It's completely immoral for a company to profit from keeping you waiting on hold...
To add my own experiences...
If you want to contact many insurance companies about anything to do with your policy, they expect you to call an 0870 number, yesinsurance are particularly bad, they will auto renew your policy unless you pay to call them and spend quite some time on hold before telling them you don't want it renewed...
And Barclays, despite offering an 0800 complaints line, will call you back to discuss the complaint, but if you don't answer they will leave a voicemail asking you to call an 0870 number... Paying for the privilege of complaining? Ridiculous...
If you don't call back, the complaint is simply discarded.