Give her a cordless phone with hand-free function.Then, she can answer the phone by pressing only one button in any concern of your home. And, she can answer the phone even she holds your son.
Besides, let her set up the habbit to pick up phone. Let her answer all the phone even you are at home.
she is at room or she is talking on your home phone, my maid will not answer the phone on first time when I phone home when she is using the phone, but she will call back later.