Having clients call your clinic isn't annoying - its part of the veterinary business. We are a service oriented field, and your staff should be well trained in communicating with clients over the phone and make an appointment. If people are looking at coming to you for routine stuff, the first phone call they make to your clinic can be a turning point in how they "feel" about you - if you make a good impression, they may come to you even if you are more expensive than the next clinic. It also helps determine critical information about the patient (which might not seem so critical for the owners!) and help the nurses/receptionists/techs "triage" the appointment - ie, it is ok for someone to make an online booking for a vacc appt in a weeks time, but not if the dog has a prolapsed globe! etc.