Print advertising, electronic advertising, promotions, jingles, word of mouth - all help to get potential customers to call, visit or click. Just for now, let's consider those who call - and get placed on hold. What do they hear? Nothing? The radio playing a competitor's advertisement? Messages completely unrelated to what was presented to get them to call? How does that help the customer decide that your product or service is the one he or she should purchase?
Let the caller hear about your promotions, your specials, what sets your organization apart from the competition, your unique selling proposition? The telephone is a touch point that will support your marketing and sales efforts, if you let it. The caller is captive. Why not use the caller's time on hold to educate, enlighten, inform, and/or influence her/him in your favor?