If they can't help, have them ask the nature of the call. You can then prepare for your return call. Call back the next day without fail. Don't neglect your phone messages. It's like throwing referral business down the drain.
Many clients go into shock when they see their invoice.
Even large corporate clients sometimes reel at the sight of their legal bills. Clients are often skeptical of hourly billings. They usually think the work could have been done in fewer hours. The best way to overcome this is to be sure the itemization is adequately detailed. Clients get less perturbed when they know exactly what they are paying for.