The first step is to try to understand this complex problem. When you are on the phone it is as if you are away from your child. You tune out your environment when you talk on the phone. They can feel lost or confused because Mom or Dad aren't there to help. Children need to keep in contact with their parents.
Parents can help by reaching a compromise with their child.
First of all accept that a toddler genuinely needs your attention and can't cope with you on the phone. Keep calls short when your child is with you. Don't expect any understanding or good behavior because it will not be there. Try to keep some activities available for the times you are on the phone. High-interest toys, crayons, blocks, or waterplay at the kitchen sink are all quiet activities that you and your child can do while you are talking on the phone.