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.
The client does not really care about your access. They care about a job done well. Become respected like your doctor, dentist, or attorney. Limit the instant access you grant to people. Don't be fooled by the old access model of customer service for real estate.