What are the disadvantages of pre-paid calling card services?

[RTuesday]

Originally Posted by DRNewcomb

2. When you're out of minutes you have to top-up.

That's also a positive - you can never get a surprise bill. If you have a month of very heavy usage, you might have to refill several times (so you're aware of it and can maybe make changes), rather than just getting a large bill weeks later.