You have to fight the urge to take care of clients in those times. Set specific times to return calls. Most of the calls we get are not important. They are someone trying to give us what they deem as urgent. They are rarely important and rarely must be handled now.
Most calls can wait a few hours to deal with.
Set specific times when you return calls. I would suggest once in late morning and once toward the end of the day. Tell people you are in appointments and you will be retuning calls at those specific times. You need to separate the concept of access from customer service. Customer service is about getting the job done well.