Amelia Boyle, known as Millie, picked up the ringing mobile and answered the call when her mother Elizabeth, 20, fainted in the hall of their home.
Linda Wright, Millie's grandmother, was on the other end of the line and heard the baby say "Who's that?" and "Mama".
The girl is not allowed to play with the mobile so Mrs Wright knew something was wrong with Millie's mother, who has a history of fainting and fitting, and dialled 999.
Miss Boyle, whose phone it was, woke up in her home in Braintree, Essex, to hear police and ambulance crews at her door.
She became confused and told them she had not called them so the officers broke down the door to get in and help.
A few minutes later three paramedics arrived and examined her. She did not need hospital treatment.
Miss Boyle said: "I went upstairs to get Millie some clothes. I came downstairs again and I was going to get her dressed up as normal.
"She was crawling around playing with some toys when I suddenly fainted between the hallway and the living room blocking the stairway.
"My mum phoned me back on the home phone and when she didn't get through, she tried on the mobile, then the home phone again."
She added: "I am so lucky to have Millie.
"Millie does not seem to be fazed by it. She must have watched me open the phone - she is not usually allowed to play with it because she tends to stick it in her mouth.
"She is a very clever little girl. I was proud of her anyway but now she is even more special."
Mrs Wright said: "If Millie hadn't answered the phone, I wouldn't have known what had happened."