If you have a mobile phone just for the sake of emergency, you probably make very few calls. Most mobile phone plans would not suit your purpose. You would be overspending.
The cheapest one is from Fido, $10 a month. It is prepaid airtime. Even though each call costs 30 cents, you would probably end up with a lot of money in the account after a year and you would need to renew each month to avoid losing it.
What is the solution?
Use SpeakOut from 7-Eleven. You buy time for $25 and it expires in a year. So if you barely make or receive phone calls, you would be spending at most $25.