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 often from someone trying to give us what they deem as urgent. Rarely must these calls be handled immediately.
Most calls can wait a few hours.
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.