We were baby sitting today and it was time for the baby to go down for a nap upstairs. We planned on being downstairs and I wanted to be able to hear if the baby was crying or what ever. Since we don't have a baby monitor I took one of our mobile phones and called my phone to create an audio connection. I then muted the microphone on my phone and placed the unmuted phone in the room with the baby. This gave us a make-shift baby monitor. I hear her talk a little before she went to sleep and then heard her talking when she woke up. Success!
All you need to do the same is two mobile phones, preferably on the same mobile network. I know I am probably not the first person to discover this, but I thought I would share anyway. Enjoy.