What kind of 'feedback' is the most user friendly / least annoying to show to the telephone caller than they are held in a telephone queue ?
Verbal message (repeated endlessly) ?
Music + sporadic verbal message ?
Ringing tone ?
Just music ?
And if music is used, is 'real' music (pop or classical) more or less annoying than a simplified series of electronic tones ?
While designing hold message, the most important is that the content people hear has to be relevant to what they are calling for and ensure that you understand the wait time users are going to hold for and customize the message and content accordingly so that the message . Based upon that you will need to structure your hold message. I recommend this article for inputs on how to design hold messages
Get To The Point So if your message on hold script looks good on paper, or a website, then it's fine, right? Nope. Print copy is a much different beast than audio copy. Obviously, print copy appeals to the eye and can be read and re-read over and over again while skipping "non-essential" details. On Hold copy has to be approached differently. Callers don't have the advantage to self-edit. Say what needs to be said, and in a way that works for the EAR. Here's an example from a message on hold for a pest removal company:
"Moles. They can cause a lot of damage. You don't like moles, and neither do we. Let's get rid of 'em. We'll be right back to tell you how."
Know Your Average Hold Time You should know, or at least have an educated guess, about the average length of hold time your callers experience. It could be 10 seconds; it could be six minutes. If callers stay on hold for a minute or longer, don't bombard them with a non-stop recitation of all you do. There's only so much a caller can absorb. Talk to your music and message on hold company about spacing out the voiced messages in a way that gives the caller some musical breaks between topics. Short hold times call for short, single-idea paragraphs. Longer hold times give you more options.
I recommend looking at this excellent article on what to put in your hold messages 5 Best Practices for Refreshing Message On Hold Content
With regards to the music type to use, I recommend checking out this article Choosing the Right Music On Hold in 5 Steps. To quote the article
Consider your audience. Maybe you and your callers are a lot alike, and you eat the same kind of food, talk the same way, and listen to the same radio stations. Or maybe you are very different, and you prefer Opera while they love the new Lady Gaga album. The key to choosing the right hold music is knowing your callers and understanding what they want to hear.
Read through your Message On Hold. What does your script say, and how does it say it? Is your writing lighthearted? Reassuring? Soothing? Exciting? Whatever music you choose should reflect your MOH content.
