Whilst you come across as a reasonable person, and your sentiments seem genuine, the total cost for making a call to an 0870 number is excessive, your company may not be raking in all of it but the company you lease the number from is, and it is quite likely that except from an initial outlay, it costs you nothing to run those lines (not including staff wages), so the benefit is not just the pittance you receive but the saving of no business rate line rental.
Anytime I see an 0870 number for a service that I need I avoid it like the plague as I see it as greed, this may be an unjustified point of view, as judging by your post some companies have a legitimate reason for using it, but the damage has already been done by your predecessor's and can't easily be undone.
That aside, well done for binning the auto queue.