"We'll call you right back," or even worse, "we'll call you back at our earliest convenience."
As a customer, these are some of my least favorite things to hear. My time is valuable. If I have used some of my time to call and inquire about a business, I expect to be able to get an answer during my first phone call. Especially in a competitive services business (like the fitness business that we work in), I probably found the company's phone number on Google/Yahoo. That means there are competitors' phone numbers on the same search. If I don't get the answer I'm looking for with my first call, then it's easy just to keep dialing the next number until I can get the answer I need.
Voicemail is the equivalent of "We'll call you back at our earliest convenience." When given the opportunity to leave a voicemail, 75% of callers will simply hang up, so I'm clearly not alone in my thinking. Each business has it's own breakdown of opportunities from phone traffic, but here's the business I know (fitness): For every 10 calls made to a typical small gym, 5 will be sales opportunities, 4 will be current members and the other 1 will be a sales call (someone calling to sell the gym cleaning wipes or similar products). By extension, that would mean that for every 10 callers who get your voicemail (remember, 75% just hang up), a typical gym is likely missing out on 7 callers, of which 4 were likely sales opportunities, and the other 2-3 are current members with a question or concern.
Customer service can be clearly impacted by missing those calls. A current customer who is calling, probably is calling because they have an immediate issue they want to get solved. Having to wait for a phone call back (even if that call comes within an hour) just adds to their frustration. Playing "phone tag" with a company can make them downright angry. Likewise, a customer who calls and is able to have their concern resolved on the first call will have a much more positive feeling towards the business.
Even more deadly are the missed sales opportunities, because those folks DO NOT WAIT for a call back. Unless you are the only business in town which can provide them your particular product or service, they will just dial the next competitor. Even worse, the customers who DID leave a voicemail are probably also calling your competitors. If they left a voicemail with you, but the competitor offered a comparable price AND picked up the phone on the first call, your competitor probably just won themselves a new customer.
Don't help your competitor. Do everything you can to make sure that you answer each call made to your business and that you can resolve any call WITHOUT having to make a call back to the customer.