Hmm there wasnt a voting option for "havent used one personally, but used to sell them for a living (amongst other phone/data services)"...
I cant think how you'd miss out such an obvious option Chris !!
OK, obviously freefone numbers are not FREE totally. There are lots of providers of freefone numbers but, for ease of example lets say you got one from BT.
Heres how its supposed to work.
Mrs Jones calls you from their BT line to your 0800 number. The call costs them nothing, and they book a disco with you - Terrific. At the end of the billing period, your bill comes in and that 5 minute call to you cost you 5 x 10p...still terrific - you got a £200~£300 booking, and it only cost you 50p in "freefone" telephone charges.
However...
Lets try that again with a little (synical) reality applied ..(even though I cant spell Synical

)
Mrs Jones calls your freefone number from her mobile. WHAMMO ... fell at the first... with one or two exceptions, a call from a NON-BT telephone to a BT landline is chargable to the caller (Mrs Jones). And wouldnt you just know it...her monthly cellular bill will be read by her, the morning of the booking...

Her telephone company will say that the charge is legitimate as their equipment/network was successfully used for X minutes to converse with the dialled number. And guess what...BT will still bill you for the same X minutes because the freefone service which you rented was ALSO successfull at handling/routing the call to your normal number... OK, you're no worse off than in the above example, but Mrs Jones is. Also as her cellular package has "free" minutes built-in, but only for geographic located numbers eg: 01256 but not non-geographic numbers like 0845, 07xxx oh and of course 0800, she would actually have been able to call you FREE from her mobile to Congleton...
Also, in reality, Mr Jones could well be calling you 3 or 4 times (from her mobile of course) in that time between the first "availability" call, and the actual function...reading out her music lists, the address and contact details for the venue, how nice her daughters outfit is...thats a couple of pounds added to your freefone usage, just on fashion tips.
But of course, business is business...Mrs Jones may well have run up £3~£4 on your freefone usage, as well as her mobile to call your "free" number, but hey, you're getting £250+++ from it, and its tax-deductable so, heck, what the hey, you're spending a little, to earn alot...good business that.
But of course, you've been advertising the freephone number all over the place. And some people know that you're also a DJ equipment Supplier. "What are the Constella 5G's like?", "Have you got Snow Fluid?", "What bulb do I need for my moonflower...I aint got the model number, urm its black, metal and rectangular". They might have called from their landline, they might have called from their mobiles (£!) but a pound to a penny (good sale price that), if they knew your freefone number, they used it to call you up, without a £250+++ booking at the end of it.
Of course to you, the normal desk phone rung, and you picked it up. You wouldnt have known whether you've been talking ohms law with a customer (or chatting up that nice girl in the Gemini outbound sales team) at their expence, or yours. The only "new" angle on this is SOME service providers of Freefone numbers can make your phone ring differently if the call is routed via the freefone number, that way, you know to get them off the line quick, as its your freefone charges that are clocking up...
Hmmm, i might give you a call tomorrow to discuss the weather... eg: Weather or not you're still thinking about a Freefone number...