Sorry for persevering, but in the precise example of car servicing, it really is to my asdvantage to answer the call.
Such calls are generally made by the service manager, who has some tchnical knowledge, and is able to “talk turkey” with regards to the finer points of vehicle mechanics. If his call goes to voicemail and I have to call back, I’ll end up dealing with some receptionist who’s skill set is largely restricted to smiling sweetly and offering cups of coffee.
Another example is GP access. Our own surgery allows telephone appointments whereby a doctor will call you back within an agreed time window. The system works well as long as I answer the call, but I don’t think the doctor would take kindly if I refused to answer, just because his call didn’t show the main reception number, or was withheld.