I can sympathize with UA on this one. It's not that it's unreasonable of the client to just quickly check that it's worth calling straight back, but I do find that once you answer one question, they don't stop. All of a sudden it's "wait, wait, wait, just one more question while I've got you on the line" and after that you can't get rid of them. Then you're in the position of having to either continue with the conversation, or politely tell them that you're going to hang up, and will be happy to speak to them when they call back with a visible number - and nobody likes to be hung up on, whether they're genuine or a timewaster. Shouting is probably not the way forward, but I think it makes life a lot easier if you immediately say, in a polite and friendly way, " I'd be more than happy to discuss my availabilty with you, but I'm afraid you will need to call back with a visible number".