For Canada, in case you're interested, 7-11 has Speak Out prepaid. You buy an unlocked Nokia phone and any amount you pay is good for 365 days and the fee is 25 cents per minute local. Your local area is defined as where the phone's area code is located. For the US I have the same phone from T-Mobile and top it up with $10 amounts when I need it and it's good for 90 days as others have noted. Even if it has expired, I don't lose the number. After $100 the expiry goes to one year.