In my opinion, there's no excuse to call from a withheld number unless you're at a hotel you're unfamiliar with and you're unaware. I'm a bit skeptical when people tell me that they didn't realise that their number was withheld. People at work must realise how their exchange works. Surely they've used their work number enough times to know this.
I've had nervous gents call me showing their number. They either chicken out and don't confirm, or they do call back and say that they're too nervous to go through with the booking, which is fair enough. I appreciate that they took the time to tell me. And if a customer's really that nervous, that's what email's for. You don't have to "confront" a voice, and you're less nervous writing an email. That just makes more sense. I do appreciate gents can be nervous. But another way around it is to call from a withheld number if they have to, leave a brief message explaining their situation and why they would prefer their number withheld, then give me a time they're going to call back so I can answer the phone, or call within the next five minutes. Or they need to finally realise that their phone call's not going to be answered by me unless they show their number. Simple really.
I worked from the back of the local papers in the early days. I have never experienced so many timewasters and withheld numbers in such a short space of time, to the point where I have a deep aversion for both.