If they called me while we were in school, I talked with them for a few seconds and then told them that we were in school. I then asked them if I could call them back and set up a specific time to return their call. Now that I have older children, I try never to answer the phone during school hours. If someone calls, I have the children take the caller's name and number, and I return the call as soon as we finish school. If I do answer the phone, I keep the call as brief as possible by making arrangements to return the call later if necessary.
Preschoolers are another story.
I will be devoting a later article solely to the subject of dealing with preschoolers. In the meantime, here are some things that worked for me. Each day before you begin school talk to your preschoolers about what they are going to do while you are in school. Tell them that if they change activities they must tell you first. Put the oldest preschooler in charge of the others.